libstdc++: Do not require effective target pthread for some tests

We can remove { dg-require-effective-target pthread } and allow these
tests non-pthread targets and conditionally adding -pthread only for
pthread targets.

Also remove the { dg-options "-std=gnu++20" } that is no longer needed.

libstdc++-v3/ChangeLog:

	* testsuite/29_atomics/atomic_ref/wait_notify.cc: Do not
	restrict to pthreads targets.
	* testsuite/30_threads/jthread/3.cc: Likewise.
	* testsuite/30_threads/semaphore/100806.cc: Likewise.
	* testsuite/30_threads/semaphore/try_acquire.cc: Likewise.
	* testsuite/30_threads/semaphore/try_acquire_for.cc: Likewise.
	* testsuite/30_threads/semaphore/try_acquire_posix.cc: Likewise.