you can set BUILD_CC="${CHOST}-clang" in the build files like we do with CC today

This is the portage-stable list:

./eclass/linux-mod.eclass:			eval "emake HOSTCC=\"$(tc-getBUILD_CC)\" \
./eclass/toolchain-funcs.eclass:tc-getBUILD_PROG() { _tc-getPROG CBUILD "BUILD_$1 $1_FOR_BUILD HOST$1" "${@:2}"; }
./media-video/mplayer/mplayer-1.0_rc4_p20120405.ebuild:		--host-cc="$(tc-getBUILD_CC)" \
./dev-util/ninja/ninja-1.7.1.ebuild:		local -x AR=$(tc-getBUILD_AR)
./app-emulation/qemu/qemu-2.6.0.ebuild:		--host-cc="$(tc-getBUILD_CC)"
./sys-firmware/seabios/seabios-1.8.2.ebuild:		HOST_CC="$(tc-getBUILD_CC)" \
./sys-firmware/ipxe/ipxe-1.0.0_p20130624.ebuild:			HOST_CC=$(tc-getBUILD_CC) \
./app-crypt/gnupg/gnupg-1.4.15.ebuild:		CC_FOR_BUILD=$(tc-getBUILD_CC) \
./sys-apps/iproute2/iproute2-4.2.0.ebuild:		-e "/^HOSTCC/s:=.*:= $(tc-getBUILD_CC):" \
./sys-apps/toybox/toybox-0.7.2.ebuild:	export HOSTCC=$(tc-getBUILD_CC)
./sys-apps/busybox/busybox-1.26.2-r1.ebuild:		-e "/^HOSTCC/s:=.*:= $(tc-getBUILD_CC):" \
./sys-libs/e2fsprogs-libs/e2fsprogs-libs-1.43.4.ebuild:	BUILD_CC="$(tc-getBUILD_CC)" \
./sys-libs/timezone-data/timezone-data-2017b.ebuild:			CC="$(tc-getBUILD_CC)" \
./dev-libs/libchewing/libchewing-0.3.2-r1.ebuild:	export CC_FOR_BUILD="$(tc-getBUILD_CC)"

you could try adding a make.defaults file to features/llvm/make.defaults that sets up the BUILD_xxx variables

if it causes a problem with the sdk, look at Caroline's CLs:
  https://chromium-review.googlesource.com/481864
  https://chromium-review.googlesource.com/503509

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/ddd28fec8022a674cbc48889d31272fb95cb112e

commit ddd28fec8022a674cbc48889d31272fb95cb112e
Author: Manoj Gupta <manojgupta@google.com>
Date: Sun Sep 17 00:55:10 2017

cros_use_gcc: set BUILD_{CC|CXX} variables to gcc.

BUILD_{CC|CXX} must also be set to gcc versions when forcing to
gcc is desired for a package.
This CL is required to land CL:669301

BUG= chromium:748476 
TEST=chromiumos-sdk {daisy/kevin/caroline}-release tryjobs passed.

Change-Id: I7f43cf16993844b43bdab01115bf1f35622173c5
Reviewed-on: https://chromium-review.googlesource.com/669864
Commit-Ready: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>

[modify] https://crrev.com/ddd28fec8022a674cbc48889d31272fb95cb112e/profiles/base/profile.bashrc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/7a2cae2e9ee688d7a3b326167a5c7de3e53b1a8a

commit 7a2cae2e9ee688d7a3b326167a5c7de3e53b1a8a
Author: Manoj Gupta <manojgupta@google.com>
Date: Mon Sep 18 19:03:03 2017

glibc: Call cros_use_gcc inside eblit-src_configure-pre.

glibc needs to be built with gcc. It also requires
that TARGET_CC is also set to gcc. To ensure that, call
cros_use_gcc in eblit-src_configure-pre.
Alternative to modifying the build is to create config/env/cross-*/glibc files
just to create a call to cros-use_gcc which will be a mess to manage.

BUG= chromium:748476 
TEST=chromiumos-sdk passed.

Change-Id: I80dc95266b7bc01c7c43bfa450092b1aae1deaf8
Reviewed-on: https://chromium-review.googlesource.com/669925
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Trybot-Ready: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>

[rename] https://crrev.com/7a2cae2e9ee688d7a3b326167a5c7de3e53b1a8a/sys-libs/glibc/glibc-2.23-r10.ebuild

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/crosutils/+/9f51d2ad382d22c6f2305f57ee7bdfcaf972bacd

commit 9f51d2ad382d22c6f2305f57ee7bdfcaf972bacd
Author: Manoj Gupta <manojgupta@google.com>
Date: Wed Sep 20 10:39:25 2017

Create BUILD_CC/BUILD_CXX variables in SDK.

This CL is to enable getting clang as the compiler
through tc-getBUILD_CC/CXX calls.

Does not impact current chroot. This CL only creates
the variables which are unused for now.

BUG= chromium:748476 
TEST=BUILD_CC variables are created.
Change-Id: Ifb485d8a474f34724bb917ac702159c3e155a4d2
Reviewed-on: https://chromium-review.googlesource.com/669301
Commit-Ready: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>

[modify] https://crrev.com/9f51d2ad382d22c6f2305f57ee7bdfcaf972bacd/sdk_lib/make_conf_util.sh

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/a94d501bb423a46cbf72f579b5ac695116294352

commit a94d501bb423a46cbf72f579b5ac695116294352
Author: Manoj Gupta <manojgupta@google.com>
Date: Thu Sep 21 08:08:05 2017

Set BUILD_CC to clang as the build compiler in target builds.

Currently calling tc-getBUILD_CC in ebuild returns gcc. This created
a mixed gcc/clang compilation situation if an ebuild needs some binaries
to be built on host. Fix this by setting the BUILD_CC to the host clang
compiler.

BUG= chromium:748476 
TEST=Dumping tc-getBUILD_{CC,CXX} in emerge-caroline chromeos-chrome shows x86_64-pc-linux-gnu-clang.
TEST=cros tryjob -g 674725 {daisy,elm,kevin,pyro,eve,peppy,squawks,caroline,whirlwind,lakitu}-release passed.

Change-Id: Iab6a4871963e1b5068572db8f60b4e25ed22b08e
Reviewed-on: https://chromium-review.googlesource.com/674725
Commit-Ready: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[add] https://crrev.com/a94d501bb423a46cbf72f579b5ac695116294352/profiles/features/llvm/make.defaults