[PATCH] Enable EHABI by default on non-Darwin ARM targets

[Renato Golin]

  I traced back the problems on MCJIT:

   1. two relocations needed recognizing (NONE, PREL31), which could easily be implemented by ignoring NONE and matching PREL31 with existing similar relocations (I need to make sure this is right)
   2.  tests failing should have nounwind on them, since there's nothing about EH on any of them. We should make sure those functions fall into "if (!Asm->MF->getFunction()->needsUnwindTableEntry())" case. I'll work on that now.
   3. '__aeabi_unwind_cpp_pr0' which could not be resolved. When JITing C++ code with exceptions, we need to make sure the right libraries are also loaded. I have no idea how to make that.

  In theory, just fixing the problem 1 and 2 above should make the tests pass, but if we add C++/EH tests, we need to make sure the libs are loaded.

http://llvm-reviews.chandlerc.com/D2627