Disable FP contraction on -Fast bot.

It's more annoying than helpful to have GCC turn mul,add into fma.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2780

CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-Fast-Trybot

Change-Id: I63f4615f73aed112f10f6cb516d899b820918298
Reviewed-on: https://skia-review.googlesource.com/2780
Commit-Queue: Cary Clark <caryclark@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
2 files changed