https://boringssl-review.googlesource.com/c/boringssl/+/24945 was mistaken in
that it thought that these AVX-512 assembly extensions were an
instruction-level thing, whereas they actually appear to be an argument-level
modifier.

This change parses them as such and unbreaks some AVX-512 instructions that can
be emitted by compilers with certain combinations of flags.

Change-Id: I9af5a4fec21f55d3198a248c9175252e229c355a
Reviewed-on: https://boringssl-review.googlesource.com/c/32484
Commit-Queue: Adam Langley <agl@google.com>
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>