Prevent internal aligned_alloc clashing with libc version

Prevent internal aligned_alloc clashing with libc version

If configure fails to detect aligned_alloc we will try to define our
own in new_opa.cc but that could clash with the libcversion in
<stdlib.h>. Use a namespace to keep them distinct.

	* libsupc++/new_opa.cc (aligned_alloc): Declare inside namespace to
	avoid clashing with an ::aligned_alloc function that was not detected
	by configure.

From-SVN: r263409