Add a special ARM trap encoding for NaCl.
More details in this thread: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20130128/163783.html

Patch by JF Bastien



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173943 91177308-0d34-0410-b5e6-96231b3b80d8