Move target_address_t to arch.h, rename to arch_addr_t

Note that the placement under arch.h is currently conceptual only.  Since
no backend defines ARCH_HAVE_ADDRESS_TYPES, it's actually in sysdep.h in
all cases.
diff --git a/sysdeps/linux-gnu/arm/breakpoint.c b/sysdeps/linux-gnu/arm/breakpoint.c
index 5bcf86e..5748401 100644
--- a/sysdeps/linux-gnu/arm/breakpoint.c
+++ b/sysdeps/linux-gnu/arm/breakpoint.c
@@ -95,7 +95,7 @@
 arch_breakpoint_init(struct Process *proc, struct breakpoint *sbp)
 {
 	/* XXX That uintptr_t cast is there temporarily until
-	 * target_address_t becomes integral type.  */
+	 * arch_addr_t becomes integral type.  */
 	int thumb_mode = ((uintptr_t)sbp->addr) & 1;
 	if (thumb_mode)
 		sbp->addr = (void *)((uintptr_t)sbp->addr & ~1);