libstdc++: Add options for libatomic to test

libstdc++: Add options for libatomic to test

This fixes a linker error on AIX:

FAIL: 30_threads/semaphore/try_acquire_posix.cc (test for excess errors)
Excess errors:
ld: 0711-317 ERROR: Undefined symbol: .__atomic_fetch_add_8
ld: 0711-317 ERROR: Undefined symbol: .__atomic_load_8
ld: 0711-317 ERROR: Undefined symbol: .__atomic_fetch_sub_8
ld: 0711-345 Use the -bloadmap or -bnoquiet option to obtain more information.
collect2: error: ld returned 8 exit status

libstdc++-v3/ChangeLog:

	* testsuite/30_threads/semaphore/try_acquire_posix.cc: Add
	options for libatomic.