ada: Add size clause to System.Address

ada: Add size clause to System.Address

Standard'Address_Size is the value provided by the code generator for the
size of pointers, and it is set as the default size of every thin pointer
by the front-end.  Now it is documented in the GNAT RM as having the value
of System.Address'Size, which is indeed the case on (correctly configured)
platforms where pointers contain exactly the number of bits that are needed
to address the memory space.

However, on platforms where pointers contain additional bits of metadata,
it has a larger value and the documented relation does not hold, which also
means that unchecked conversions between System.Address and pointers are
seen as potentially problematic.  In order to fix the discrepancy on these
platforms, this change adds the obvious size clause to System.Address, which
is confirming on all the other (correctly configured) platforms.

gcc/ada/

	* libgnat/system-aix.ads (Address): Likewise.
	* libgnat/system-darwin-arm.ads (Address): Likewise.
	* libgnat/system-darwin-ppc.ads (Address): Likewise.
	* libgnat/system-darwin-x86.ads (Address): Likewise.
	* libgnat/system-djgpp.ads (Address): Likewise.
	* libgnat/system-dragonfly-x86_64.ads (Address): Likewise.
	* libgnat/system-freebsd.ads (Address): Likewise.
	* libgnat/system-hpux-ia64.ads (Address): Likewise.
	* libgnat/system-hpux.ads (Address): Likewise.
	* libgnat/system-linux-alpha.ads (Address): Likewise.
	* libgnat/system-linux-arm.ads (Address): Likewise.
	* libgnat/system-linux-hppa.ads (Address): Likewise.
	* libgnat/system-linux-ia64.ads (Address): Likewise.
	* libgnat/system-linux-m68k.ads (Address): Likewise.
	* libgnat/system-linux-mips.ads (Address): Likewise.
	* libgnat/system-linux-ppc.ads (Address): Likewise.
	* libgnat/system-linux-riscv.ads (Address): Likewise.
	* libgnat/system-linux-s390.ads (Address): Likewise.
	* libgnat/system-linux-sh4.ads (Address): Likewise.
	* libgnat/system-linux-sparc.ads (Address): Likewise.
	* libgnat/system-linux-x86.ads (Address): Likewise.
	* libgnat/system-lynxos178-ppc.ads (Address): Likewise.
	* libgnat/system-lynxos178-x86.ads (Address): Likewise.
	* libgnat/system-mingw.ads (Address): Likewise.
	* libgnat/system-qnx-arm.ads (Address): Likewise.
	* libgnat/system-rtems.ads (Address): Likewise.
	* libgnat/system-solaris-sparc.ads (Address): Likewise.
	* libgnat/system-solaris-x86.ads (Address): Likewise.
	* libgnat/system-vxworks-ppc-kernel.ads (Address): Likewise.
	* libgnat/system-vxworks-ppc-rtp-smp.ads (Address): Likewise.
	* libgnat/system-vxworks-ppc-rtp.ads (Address): Likewise.
	* libgnat/system-vxworks7-aarch64-rtp-smp.ads (Address): Likewise.
	* libgnat/system-vxworks7-aarch64.ads (Address): Likewise.
	* libgnat/system-vxworks7-arm-rtp-smp.ads (Address): Likewise.
	* libgnat/system-vxworks7-arm.ads (Address): Likewise.
	* libgnat/system-vxworks7-ppc-kernel.ads (Address): Likewise.
	* libgnat/system-vxworks7-ppc-rtp-smp.ads (Address): Likewise.
	* libgnat/system-vxworks7-ppc64-kernel.ads (Address): Likewise.
	* libgnat/system-vxworks7-ppc64-rtp-smp.ads (Address): Likewise.
	* libgnat/system-vxworks7-x86-kernel.ads (Address): Likewise.
	* libgnat/system-vxworks7-x86-rtp-smp.ads (Address): Likewise.
	* libgnat/system-vxworks7-x86_64-kernel.ads (Address): Likewise.
	* libgnat/system-vxworks7-x86_64-rtp-smp.ads (Address): Likewise.