Sanitize all our #if __arm__ and convert to LIBCXXABI_ARM_EHABI.

This removes a bunch of TODOs while looking through and justifying
each #if as really __arm__ platform vs implementing the EHABI spec.

Change-Id: Ib2533c80dfbac7768c97bce82b153991effa7a33
Merge pull request #91 from awong-dev/ifs

Sanitize all our #if __arm__ and convert to LIBCXXABI_ARM_EHABI.