https://chromium-review.googlesource.com/c/620282 for building with llvm
but hitting https://bugs.llvm.org/show_bug.cgi?id=32390.

clang_rt.asan-dynamic-arm.vers: No such file or directory

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/9a2c5f1fedd7a84d70aa01c67d67f29821d7b0c0

commit 9a2c5f1fedd7a84d70aa01c67d67f29821d7b0c0
Author: Manoj Gupta <manojgupta@google.com>
Date: Wed Aug 30 14:06:30 2017

compiler-rt: Allow building with clang.

Build compiler-rt with clang. Still requires passing rtlib=libgcc
for bootstrapping and passing cmake configure stage.
Building with libgcc should have no impact on the generated builtins archive
library since no linking is done there.

Also cherry-pick the following upstream fix for build dependency issue
in ARM asan libraries.

commit 1a32c939c5eece22f3ca6cf70bd05a1527bc0970
Author: Evgeniy Stepanov <eugeni.stepanov@gmail.com>
Date:   Mon Aug 21 22:31:31 2017 +0000

    Fix ASan version list dependency in multi-arch builds.

    Fixes PR32390.

    Patch by Andrey Krayniak.

BUG= chromium:756702 
TEST=compiler-rt gets built for armv7a/aarch64.

Change-Id: I64262a2a38f317e01b32f1985e7334cfbc21c1bc
Reviewed-on: https://chromium-review.googlesource.com/620282
Commit-Ready: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>

[add] https://crrev.com/9a2c5f1fedd7a84d70aa01c67d67f29821d7b0c0/sys-libs/compiler-rt/files/cherry/1a32c939c5eece22f3ca6cf70bd05a1527bc0970.patch
[rename] https://crrev.com/9a2c5f1fedd7a84d70aa01c67d67f29821d7b0c0/sys-libs/compiler-rt/compiler-rt-5.0_pre305632-r2.ebuild