unwind: Allow the building of libunwind for MIPS.

Summary:
Currently, libunwind doesn't support MIPS. However, with this patch
we do allow the library to build, and we warn the user about the lack of
support for MIPS. Also, the dummy unw_getcontext() implementation for MIPS just
traps on function entry in order to avoid any confusion with silent/weird
failures at runtime.

This allows us to test an LLVM-based toolchain without the dependency on a
GCC toolchain. Of course, C++ exception handling and other things that depend
on stack unwinding will not work until we add a proper implementation of the
stub functions.

Reviewers: compnerd, logan

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D13160

llvm-svn: 248673