libstdc++: Relax memory ordering for default memory resource object

libstdc++: Relax memory ordering for default memory resource object

Currently pmr::set_default_resource and pmr::get_default_resource both
use sequentially consistent memory ordering. This is overkill. The
standard only requires that a call to set_default_resource synchronizes
with subsequent calls to set_default_resource and get_default_resource.

Using acquire-release for the setter and acquire for the getter is
sufficient to meet the requirement.

Reviewed-by: Thomas Rodgers  <trodgers@redhat.com>
libstdc++-v3/ChangeLog:

	* src/c++17/memory_resource.cc (set_default_resource): Use
	memory_order_acq_rel.
	(get_default_resource): Use memory_order_acquire.