configure: Add configure-time check for GCC 4.0+ atomic built-ins.

We found out earlier that GCC sometimes produces an error only at link time
for atomic built-ins that are not supported on a platform.  This actually
tries the link at configure time and should thus reliably detect whether
the atomic built-ins are really supported.

Signed-off-by: Ben Pfaff <blp@nicira.com>
test-atomic: Re-enable atomic read-write-modify tests.

This reverts commit 05d299e0ccca80736cd4438c3224540c5448a7d4 (test-atomic:
Drop atomic read-modify-write tests for the moment.) because the
test for detecting whether GCC support atomic operation built-ins has
been fixed.

Signed-off-by: Ben Pfaff <blp@nicira.com>