src: re-enable fast math on arm

Ref: https://github.com/nodejs/node/issues/1376
Ref: https://github.com/nodejs/node/pull/1398
Issue fixed in V8: https://chromium.googlesource.com/v8/v8/+/81703350bbb9923d211fe5b79e90bd458b0916e2
V8-Bug: https://code.google.com/p/v8/issues/detail?id=4019

PR-URL: https://github.com/nodejs/node/pull/2592


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>