Don't use _umul128 for Windows clang.

Windows clang lacks _umul128, but it has inline assembly so just use
that.

Change-Id: I6ff5d2465edc703a4d47ef0efbcea43d6fcc79fa
Reviewed-on: https://boringssl-review.googlesource.com/2454
Reviewed-by: Adam Langley <agl@google.com>
1 file changed