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.
