Some, very recent, versions of Clang now support `.arch`. Allow them to
see these directives with BORINGSSL_CLANG_SUPPORTS_DOT_ARCH.

BUG=39

Change-Id: I122ab4b3d5f14502ffe0c6e006950dc64abf0201
Reviewed-on: https://boringssl-review.googlesource.com/10600
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>