__xtensa_libgcc_window_spill is needed for __builtin_return_address(1),
but its libgcc implementation cannot be used as it makes a syscall.

Atomic intrinsics are used by single staging driver.
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>
Signed-off-by: NChris Zankel <chris@zankel.net>