Instead of directly checking if the target is android, check if
__safestack_pointer_address is available and configure android
to have the call. Maintain the -safestack-use-pointer-address cl::opt
in an unclean way by ignoring libcall availability.
Also add a RuntimeLibcallsInfo entry for __safestack_unsafe_stack_ptr,
similar to other special globals. Also add this unconditionally to most targets,
even though this seems contrary to reality. A few tests rely on unsupported OSes, so
leave that alone for now.
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