libstdc++: Add align_alloc attribute to aligned operator new

libstdc++: Add align_alloc attribute to aligned operator new

The aligned versions of operator new should use the align_alloc
attribute to help the compiler.

PR c++/86878 requests that the compiler would use the attribute to warn
about invalid attributes, so an XFAILed test is added for that.

libstdc++-v3/ChangeLog:

	* libsupc++/new (operator new): Add attribute align_alloc(2) to
	overloads taking a std::align_val_t argument.
	* testsuite/18_support/new_aligned_warn.cc: New test.

Reviewed-by: Jakub Jelinek <jakub@redhat.com>