In a freestanding library we don't install the <pstl/pstl_config.h>
header, so don't try to include it unless it exists.

Explicitly declare aligned alloc functions for freestanding, because
<cstdlib> doesn't declare them.

	PR libstdc++/92376
	* include/bits/c++config: Only do PSTL config when the header is
	present, to fix freestanding.
	* libsupc++/new_opa.cc [!_GLIBCXX_HOSTED]: Declare allocation
	functions if they were detected by configure.