libstdc++: Fix memory_resource.cc bootstrap failure for non-gthreads targets

libstdc++: Fix memory_resource.cc bootstrap failure for non-gthreads targets

The new choose_block_size function added in r16-2112-gac2fb60a67d6d1 was
defined inside an #ifdef _GLIBCXX_HAS_GTHREADS group, which means that
it's not available for single-threaded targets, and so can't be used by
unsynchronized_pool_resource. Move it before that preprocessor group so
it's always defined.

libstdc++-v3/ChangeLog:

	* src/c++17/memory_resource.cc: Adjust indentation of unnamed
	namespaces.
	(pool_sizes): Add comment.
	(choose_block_size): Move outside preprocessor group for
	gthreads targets.
	* testsuite/20_util/synchronized_pool_resource/118681.cc:
	Require gthreads.