[libcxxabi] Fix -Werror build for 32-bit non-ARM.

Summary:
The inclusion of Unwind-EHABI.h was insufficiently guarded
(LIBCXXABI_ARM_EHABI was beign checked without ever being defined).

Move the check into the header file itself, add the check to the
source file, and clean up the existing checks.

LIBCXXABI_ARM_EHABI didn't have a canonical defintion; it was
duplicated across cxxabi.h, libunwind.h, and unwind.h. Move the
definition into __cxxabi_config.h and clean up the old cruft (note: we
will have to ship this header).

There are also a few drive-by formatting/whitespace cleanups.

Reviewers: jroelofs, thakis, compnerd

Reviewed By: compnerd

Subscribers: compnerd, aemerson, cfe-commits

Differential Revision: http://reviews.llvm.org/D7419

llvm-svn: 228363