libstdc++: Simplify dejagnu directives for some tests using threads

Replace dg-require-effective-target directives with a target selector on
the dg-do directive.

We can also remove { dg-require-effective-target pthread } and the
associated { dg-options "-pthread" } by allowing it to run on
non-pthread targets and conditionally adding -pthread only for pthread
targets.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/shared_ptr/atomic/3.cc: Simplify dg
	directives.
	* testsuite/20_util/shared_ptr/thread/default_weaktoshared.cc:
	Likewise.
	* testsuite/20_util/shared_ptr/thread/mutex_weaktoshared.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/allocate.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/allocate_single.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/cons.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/cons_single.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/is_equal.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/multithreaded.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/options.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/release.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/release_single.cc:
	Likewise.
	* testsuite/30_threads/condition_variable_any/stop_token/wait_on.cc:
	Likewise.
	* testsuite/30_threads/stop_token/stop_callback/deadlock-mt.cc:
	Likewise.
	* testsuite/30_threads/stop_token/stop_callback/destroy.cc:
	Likewise.