libthr: Disable stack unwinding on ARM.

When a thread exits, _Unwind_ForcedUnwind() is used to walk up stack
frames executing pending cleanups pushed by pthread_cleanup_push().
The cleanups are popped by thread_unwind_stop() which is passed as a
callback function to _Unwind_ForcedUnwind().

LLVM's libunwind uses a different function type for the callback on
32-bit ARM relative to all other platforms. The previous unwind.h
header (as well as the unwind.h from libcxxrt) use the non-ARM type on
all platforms, so this has likely been broken on 32-bit arm since it
switched to using LLVM's libunwind.

For now, just disable stack unwinding on 32-bit arm to unbreak the
build until a proper fix is tested.