util/u_atomic: Ignore warnings interlocked accesses.

util/u_atomic: Ignore warnings interlocked accesses.

These are due how we implemented the atomic tests, not the atomic
implementation itself.  It's also difficult to refactor the code to
avoid the warnings due to the use of macros -- the code would be quite
hairy.

Reviewed-by: Brian Paul <brianp@vmware.com>