The issue is mosys is also built as a host package.
But when building it as a host package, rust is not installed yet on the SDK builder.

I think moving mosys from target-chromium-os-sdk ebuild to target-chromium-os-sdk-post-cross ebuild should probably fix it.

I might just be reading things wrong, but are you sure this is the problem? I had a very similar issue (regarding armv7a-cros-linux-gnueabihf instead of armv7a-cros-linux-gnueabi) when originally trying to enable the rust version of mosys. The issue there was related to changing the name of that binary breaking rust:

https://bugs.chromium.org/p/chromium/issues/detail?id=711369

Another change related to that bug went in last night:

https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/1055211

The changes related to the bug 711369 won't have any impact on InitSDK stage in chromiumos-sdk builder.
The issue is mosys build needs cargo which pulls in rust. And rust can't be built  in InitSDK stage when bootstrapping.

I can't imagine any reason that mosys should be in the InitSDK stage at all. Let's get it out of there. That will fix the problem.

The change is approved and ready to submit:

https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/1112353

For now, nyan PreCQ is broken and has been failing for several hours. 

https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8942986117923673584

https://cros-goldeneye.corp.google.com/chromeos/legoland/builderHistory?buildConfig=nyan_blaze-no-vmtest-pre-cq&buildBranch=master

I will submit this change when I can after the PreCQ is fixed.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/1647d71552ac477f1d95a1214f3176ea84babe31

commit 1647d71552ac477f1d95a1214f3176ea84babe31
Author: Samantha Miller <samanthamiller@google.com>
Date: Sat Jun 23 03:19:35 2018

rust: move mosys to target-chromium-os-sdk-post-cross

BUG= chromium:855653 
TEST=tryjob

Change-Id: Ifde2bbafd03723860c55aa1e98b95c9e7baf3f19
Reviewed-on: https://chromium-review.googlesource.com/1112353
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Samantha Miller <samanthamiller@google.com>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>

[modify] https://crrev.com/1647d71552ac477f1d95a1214f3176ea84babe31/virtual/target-chromium-os-sdk-post-cross/target-chromium-os-sdk-post-cross-1.ebuild
[rename] https://crrev.com/1647d71552ac477f1d95a1214f3176ea84babe31/virtual/target-chromium-os-sdk-post-cross/target-chromium-os-sdk-post-cross-1-r2.ebuild
[modify] https://crrev.com/1647d71552ac477f1d95a1214f3176ea84babe31/virtual/target-chromium-os-sdk/target-chromium-os-sdk-1.ebuild
[rename] https://crrev.com/1647d71552ac477f1d95a1214f3176ea84babe31/virtual/target-chromium-os-sdk/target-chromium-os-sdk-1-r116.ebuild

no longer failing: https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8942952521347924944