[MIPS] Add missing NaCl defines for MIPS

Add missing NaCl definitions for MIPS architecture.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=2275
TEST= build MIPS Chromium with NaCl

Review URL: https://codereview.chromium.org/197213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256622 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256622