libstdc++: Add comment about memalign requirements

libstdc++: Add comment about memalign requirements

The memalign man page on Solaris and QNX lists an additional requirement
for the alignment value that is not present in all implementation of
that non-standard function. For both those targets we should actually be
using posix_memalign anyway, so it doesn't matter. This just adds a
comment making note of that fact.

libstdc++-v3/ChangeLog:

	* libsupc++/new_opa.cc (aligned_alloc): Add comment.