misc: Enable tst-atomic for clang

The atomic.h macros now uses compiler builtins.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

misc/tst-atomic.c

@@ -16,7 +16,6 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#ifndef __clang__
 #include <stdio.h>
 #include <atomic.h>
 
@@ -253,12 +252,4 @@ do_test (void)
 }
 
 #include <support/test-driver.c>
-#else
-#include <support/test-driver.h>
 
-int
-main (void)
-{
-  return EXIT_UNSUPPORTED;
-}
-#endif