Explicitly link with libatomic when needed.

Explicitly link with libatomic when needed.

	2020-02-29  John David Anglin  <danglin@gcc.gnu.org>

	* testsuite/30_threads/stop_token/stop_callback.cc: Add libatomic
	option.
	* testsuite/30_threads/stop_token/stop_callback/deadlock-mt.cc:
	Likewise.
	* testsuite/30_threads/stop_token/stop_callback/deadlock.cc: Likewise.
	* testsuite/30_threads/stop_token/stop_callback/destroy.cc: Likewise.
	* testsuite/30_threads/stop_token/stop_callback/invoke.cc: Likewise.
	* testsuite/30_threads/stop_token/stop_source.cc: Likewise.
	* testsuite/30_threads/stop_token/stop_source/assign.cc: Likewise.
	* testsuite/30_threads/stop_token/stop_token.cc: Likewise.
	* testsuite/30_threads/stop_token/stop_token/stop_possible.cc:
	Likewise.