util/u_atomic: provide 64bit atomics where they're missing

util/u_atomic: provide 64bit atomics where they're missing

There are still some distributions trying to support unfortunate people
with old or exotic CPUs that don't have 64bit atomic operations. When
compiling for such a machine, gcc conveniently inserts a library call to
a helper, but it's implementation is missing and we get a linker error.
This allows us to provide our own implementation, which is marked weak
to prefer a better implementation, should one exist.

v2: changed copyright, some style adjustments
v3: [mattst88] Print results with AC_MSG_CHECKING/AC_MSG_RESULT

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93089
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>