RT#4530

(Imported from upstream's 7123aa81e9fb19afb11fdf3850662c5f7ff1f19c.)

We've yet to enable this code, but this confirms that we do indeed need
to get our future all-variants stuff working on Windows as well as
Linux and find an AVX2-capable CI setup on each.

The crash here is caused by some win64-only code using %rax as a frame
pointer (perlasm injects a mov rax,rsp in the prologue of every win64
function).

Change-Id: Ifbe59ceb6ae29266d9cf8a461920344a32b6e555
Reviewed-on: https://boringssl-review.googlesource.com/10366
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>