libstdc++: Make std::assume_aligned a constexpr function [PR 97132]

libstdc++: Make std::assume_aligned a constexpr function [PR 97132]

The cast from void* to T* in std::assume_aligned is not valid in a
constexpr function. The optimization hint is redundant during constant
evaluation anyway (the compiler can see the object and knows its
alignment). Simply return the original pointer without applying the
__builtin_assume_aligned hint to it when doing constant evaluation.

This change also removes the preprocessor branch that works around
uintptr_t not being available. We already assume that type is present
elsewhere in the library.

libstdc++-v3/ChangeLog:

	PR libstdc++/97132
	* include/bits/align.h (align) [!_GLIBCXX_USE_C99_STDINT_TR1]:
	Remove unused code.
	(assume_aligned): Do not use __builtin_assume_aligned during
	constant evaluation.
	* testsuite/20_util/assume_aligned/1.cc: Improve test.
	* testsuite/20_util/assume_aligned/97132.cc: New test.