support: add support_address_diff function

support: add support_address_diff function

Some malloc tests compare pointers meaning to compare addresses.
On AArch64, the 64-bit value of the pointer may contain metadata
along with the values of the address.

In order to correctly compare addresses, we add new function for
AArch64 target that will use the AArch64 64 SUBP (subtract pointer)
instruction when it is available. This instruction uses the 56-bit
addresses ignoring top-byte metadata.

Best implementation is selected using ifunc resolver.

On other targets and also on AArch64 when MTE is not available this
function defaults to PTR_DIFF defined in libc-pointer-arith.h.

Three malloc tests are modified accordingly:
 - tst-memalign-2.c
 - tst-memalign-3.c
 - tst-realloc.c

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>