src: disable fast math on arm

The "fast" implementation of Math.exp() and Math.tanh() emits ARMv7
movt/movw instructions on ARMv6 (notably, the original Raspberry Pi.)

Disable fast math for now.  The adventurous can enable it again with
the --fast_math switch.

PR-URL: https://github.com/iojs/io.js/pull/1398
Refs: https://github.com/iojs/io.js/issues/1376


Reviewed-By: Roman Reiss <me@silverwind.io>
V8-Bug: https://code.google.com/p/v8/issues/detail?id=4019