Only use custom SSE FMUL and FMAC with non-clang compilers.

Only use custom SSE FMUL and FMAC with non-clang compilers.

clang's auto-vectorized C version performs better according to the
Chrome Performance Dashboard.  Searching back through the logs, this
occurred when we switched over to clang by default.

We could try to microoptimize further, but it's less of a maintenance
burden to just let the compiler do its thing!

The main reason the clang version is faster is it does 2x 128bit
operations per loop. Simply copying these optimization yields ~97%
similar performance, but  the SIMD code a bit gnarlier. As such I
choose to simply use the C variant when clang is present.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/599693002

Cr-Commit-Position: refs/heads/master@{#297268}