util/u_atomic: Add new macro p_atomic_add

util/u_atomic: Add new macro p_atomic_add

This provides for atomic addition, which will be used by an upcoming
shader-cache patch. A simple test is added to "make check" as well.

Note: The various O/S functions differ on whether they return the
original value or the value after the addition, so I did not provide
an add_return() macro which would be sensitive to that difference.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>