Make _Unwind_Backtrace() work on ARM.

Summary: Since the personality functions do the actual unwinding on ARM,
and will also stop unwinding when they encounter a handler, we invoke
_Unwind_VRS_Interpret() directly form _Unwind_Backtrace().

To simplify, the logic for decoding an EHT is moved out of
unwindOneFrame() and into its own function, decode_eht_entry(). Unlike
unwindOneFrame(), which could only handle ARM's compact personality
function entries (section 6.3) decode_eht_entry() can handle the generic
entries (section 6.2).

Reviewers: jroelofs

Reviewed By: jroelofs

Subscribers: piman, aemerson, cfe-commits

Differential Revision: http://reviews.llvm.org/D5112


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@216730 91177308-0d34-0410-b5e6-96231b3b80d8