MIPS[64]R6 supports only fusion multiply-accumulate instructions, and using
these causes failures of several tests that expect exact floating-point
results. Therefore we disable fusion multiply-accumulate in both emitted and
compiled code on R6.

TEST=cctest/test-run-machops/RunFloat64MulAndFloat64Add1,mjsunit/es6/math-expm1.js
mjsunit/es6/math-fround.js,mjsunit/compiler/multiply-add.js

BUG=

Review-Url: https://codereview.chromium.org/2569683002
Cr-Commit-Position: refs/heads/master@{#41717}