ARM: Try to avoid VMSR instruction and drop redundant VCVT

We were doing a redundant VCVT operation in MacroAssembler::EmitECMATruncate.
Also, setting the FPSCR exception flags is expensive on some CPUs, wo we should
try to avoid it if we can.

Thanks to Rodolph Perfetta for the input on this!

Review URL: https://chromiumcodereview.appspot.com/12217014
Patch from Hans Wennborg <hans@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13601 ce2b1a6d-e550-0410-aec6-3dcde31c8c00