[PATCH] Implement new hook for max_align_t_align

[Jason Merrill]

/* Alignment, in bits, a C conformant malloc implementation has to
provide.
   The HP-UX malloc implementation provides a default alignment of 8
bytes.
   This can be increased with mallopt.  The glibc implementation also
provides
   8-byte alignment.  Note that this isn't enough for various POSIX
types such
   as pthread_mutex_t.  However, since we no longer need the 16-byte
alignment
   for atomic operations, we ignore the nominal alignment specified
for these
   types.  The same is true for long double on 64-bit HP-UX.  */

If PA malloc doesn't actually provide 16-byte alignment, this change
seems problematic; it will mean any type that wants 16-byte alignment
will silently get 8-byte alignment instead.

Jason