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/trace.c b/sysdeps/linux-gnu/trace.c
index cef8e3d..0829bdb 100644
--- a/sysdeps/linux-gnu/trace.c
+++ b/sysdeps/linux-gnu/trace.c
@@ -782,7 +782,7 @@
 			 * reason for the re-enablement.  In that case
 			 * handle it.  */
 			if (event->type == EVENT_BREAKPOINT) {
-				target_address_t ip
+				arch_addr_t ip
 					= get_instruction_pointer(task);
 				struct breakpoint *other
 					= address2bpstruct(leader, ip);