Enable backtrace_test for ARM.

[logan]

Summary

With previous changes, I am able to run backtrace_test with
shared library build of libc++abi and libunwind.

I believe we can enable this test for ARM now!

(p.s. This is the 6/6 patch to fix the backtrace_test and libgcc support.)

[rengolin]

Hurray! When all the other patches are merged, this is obviously approved! :)

cheers,
--renato

[logan]

Thanks. Committed as rL226823.

test/backtrace_test.cpp

 //===---------------------- backtrace_test.cpp ----------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
-// XFAIL: arm
 #include <assert.h>
 #include <stddef.h>
 #include <unwind.h>
 
 extern "C" _Unwind_Reason_Code
 trace_function(struct _Unwind_Context* context, void* ntraced) {
   (*reinterpret_cast<size_t*>(ntraced))++;
   // We should never have a call stack this deep...