ARM: Fix comparison of NaN values.

Enables the cumulative exception flag when comparing values, and uses it to
detect NaN results.

BUG=1023
TEST=none

Code review URL: http://codereview.chromium.org/6142004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6236 ce2b1a6d-e550-0410-aec6-3dcde31c8c00