x86, bitops: note on __test_and_clear_bit atomicity

__test_and_clear_bit is actually atomic with respect
to the local CPU. Add a note saying that KVM on x86
relies on this behaviour so people don't accidentaly break it.
Also warn not to rely on this in portable code.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>