libstdc++: Do not use C++11 alignof in C++98 mode [PR104395]

libstdc++: Do not use C++11 alignof in C++98 mode [PR104395]

When -faligned-new (or Clang's -faligned-allocation) is used our
allocators try to support extended alignments, gated on the
__cpp_aligned_new macro. However, because they use alignof(_Tp) which is
not a keyword in C++98 mode, using -std=c++98 -faligned-new results in
errors from <memory> and other headers.

We could change them to use __alignof__ instead of alignof, but that
would potentially alter the result of the conditions, because e.g.
alignof(long long) != __alignof__(long long) on some targets. That's
probably not an issue for any types with extended alignment, so maybe it
would be a safe change.

For now, it seems acceptable to just disable the extended alignment
support in C++98 mode, so that -faligned-new enables std::align_val_t
and the corresponding operator new overloads, but doesn't affect
std::allocator, __gnu_cxx::__bitmap_allocator etc.

libstdc++-v3/ChangeLog:

	PR libstdc++/104395
	* include/bits/new_allocator.h: Disable extended alignment
	support in C++98 mode.
	* include/bits/stl_tempbuf.h: Likewise.
	* include/ext/bitmap_allocator.h: Likewise.
	* include/ext/malloc_allocator.h: Likewise.
	* include/ext/mt_allocator.h: Likewise.
	* include/ext/pool_allocator.h: Likewise.
	* testsuite/ext/104395.cc: New test.