Add workaround for aligned_alloc bug on AIX

Add workaround for aligned_alloc bug on AIX

On AIX 7.2.0.0 aligned_alloc incorrectly requires the alignment to be a
multiple of sizeof(void*).

This adds a workaround to the operator new overload taking an alignment
value, to increase the alignment (and size) if needed.

Backport from mainline
2018-07-30  Jonathan Wakely  <jwakely@redhat.com>

	* libsupc++/new_opa.cc (operator new(size_t, align_val_t)): Add
	workaround for aligned_alloc bug on AIX.
	* testsuite/18_support/new_aligned.cc: New test.

From-SVN: r263375