libstdc++: Use feature test macros in <bits/stl_construct.h>
The preprocessor checks for __cplusplus in <bits/stl_construct.h> should
use the appropriate feature test macros instead of __cplusplus, namely
__glibcxx_raw_memory_algorithms and __cpp_constexpr_dynamic_alloc.
For the latter, we want to check the compiler macro not the library's
__cpp_lib_constexpr_dynamic_alloc, because the latter is not defined for
freestanding but std::construct_at needs to be.
libstdc++-v3/ChangeLog:
* include/bits/stl_construct.h (destroy_at, construct_at): Guard
with feature test macros instead of just __cplusplus.