Revert "Allow HSW stages to detect FMAs opportunistically."

This reverts commit 670c4688.

Reason for revert:

Mysteriously, this has caused some regressions on a bunch of bots that do not support the Haswell instruction set.  Reverting just to see if that's real (and because this wasn't really a noticeable win anywhere when landing the first time).

Bug: 825647, 825586, 825646, 825104

Original change's description:
> Allow HSW stages to detect FMAs opportunistically.
>
> We used to build the .S files with -ffp-contract=fast for this, and
> adding it here to the HSW opts will have the equivalent effect.  None of
> the other x86 targets support FMAs.
>
> In many key places we do call _mm256_fmadd_ps explicitly, but in several
> other stages we have been relying on Clang to sniff out opportunities,
> especially for fused-multiply-subtracts, which we're just not very good
> at seeing in our heads.
>
> Bug: 821012
> Change-Id: I2767cbdb7151d60ea7553e83f51b28ff9340eb94
> Reviewed-on: https://chromium-review.googlesource.com/963141
> Reviewed-by: Florin Malita <fmalita@chromium.org>
> Commit-Queue: Mike Klein <mtklein@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#543262}

TBR=fmalita@chromium.org,herb@chromium.org,mtklein@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 821012
Change-Id: I61c0627270853b8acdcf6b20fca17ba0fd8bba55
Reviewed-on: https://chromium-review.googlesource.com/980293
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545897}

skia/BUILD.gn

@@ -650,7 +650,6 @@ if (current_cpu == "x86" || current_cpu == "x64") {
     }
     if (!is_win) {
       cflags = [
-        "-ffp-contract=fast",
         "-mavx2",
         "-mbmi",
         "-mbmi2",