lib: add module support to atomic64 tests

Allow to compile the atomic64 test code either to a loadable module, or
builtin into the kernel.

Link: http://lkml.kernel.org/r/1483470276-10517-3-git-send-email-geert@linux-m68k.org


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>