xtensa: provide dummy gcc intrinsics

__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: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>