That way, we don't have to use -mattr=sse2 for ARM in
cross tests, etc.

Default to NEON for now. Also put in an entry for HW
divide in ARM mode. There's bunches of features that are
possible though, e.g.,:

https://github.com/llvm-mirror/llvm/blob/master/lib/Target/ARM/ARM.td

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1191573003.