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/proc.h b/proc.h
index 0dfa7db..03ef044 100644
--- a/proc.h
+++ b/proc.h
@@ -99,7 +99,7 @@
 	/* Dictionary of breakpoints (which is a mapping
 	 * address->breakpoint).  This is NULL for non-leader
 	 * processes.  XXX note that we store addresses (keys) by
-	 * value.  That assumes that target_address_t fits in host
+	 * value.  That assumes that arch_addr_t fits in host
 	 * pointer.  */
 	Dict * breakpoints;