libstdc++: Add missing free functions for atomic_flag [PR103934]

libstdc++: Add missing free functions for atomic_flag [PR103934]

This patch adds -
  atomic_flag_test
  atomic_flag_test_explicit

Which were missed when commit 491ba6 introduced C++20 atomic flag
test.

libstdc++-v3/ChangeLog:

	PR libstdc++/103934
	* include/std/atomic (atomic_flag_test): Add.
	(atomic_flag_test_explicit): Add.
	* testsuite/29_atomics/atomic_flag/test/explicit.cc: Add
	test case to cover missing atomic_flag free functions.
	* testsuite/29_atomics/atomic_flag/test/implicit.cc:
	Likewise.

(cherry picked from commit a8d769045b43e8509490362865a85cb31a855ccf)