libstdc++: Fix test failures at -O0

libstdc++-v3/ChangeLog:

	* testsuite/20_util/monotonic_buffer_resource/allocate.cc:
	Ignore -Walloc-larger-than warning.
	* testsuite/20_util/unsynchronized_pool_resource/allocate.cc:
	Likewise.
	* testsuite/29_atomics/atomic/cons/user_pod.cc: Compile with -O1
	to avoid linker error for __atomic_is_lock_free.