debug: clang now does provide _Unwind_GetIP on ARM

Summary:
Our clang now does provide _Unwind_GetIP on ARM. Without this change,
we get the following:

external/chromium_org/base/debug/stack_trace_android.cc:41:18: error: call to '_Unwind_GetIP' is ambiguous
  uintptr_t ip = _Unwind_GetIP(context);
                 ^~~~~~~~~~~~~
prebuilts/clang/linux-x86/3.4/lib/clang/3.4/include/unwind.h:179:14: note: candidate function
_Unwind_Word _Unwind_GetIP(struct _Unwind_Context *__context) {
             ^
external/chromium_org/base/debug/stack_trace_android.cc:32:11: note: candidate function
uintptr_t _Unwind_GetIP(_Unwind_Context* context) {
          ^

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D143