MSAN is incompatible with hand-written assembly code. Previously we
required that OPENSSL_NO_ASM be set when building with MSAN, and the
CMake build would take care of this. However, with other build systems
it wasn't always so easy.

This change automatically disables assembly when the compiler is
configured for MSAN.

Change-Id: I6c219120f62d16b99bafc2efb02948ecbecaf87f
Reviewed-on: https://boringssl-review.googlesource.com/31724
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>