Improved stubs for ARM unwinding

Instead of assuming we are using compact unwinding, which seems to have
apple specific code in it, we define a new _LIBXX_SUPPORTS_ARM_UNWIND
configuration macro, and stub out at the point where choose an unwind
method. Remove the stubs inside compact-unwind-specific code.

Change-Id: I67a40a6a08dbbd45834c78e828fcad0af82be402