gdb: rename rs6000-nat.c to rs6000-aix-nat.c

This file seems to be AIX-specific, according to its contents and
configure.nat.  Rename it to rs6000-aix-nat.c, to make that clear (and
to follow the convention).

Change-Id: Ib418dddc6b79b2e28f64431121742b5e87f5f4f5

gdb/Makefile.in

@@ -2288,8 +2288,8 @@ ALLDEPFILES = \
 	riscv-ravenscar-thread.c \
 	riscv-tdep.c \
 	rl78-tdep.c \
+	rs6000-aix-nat.c \
 	rs6000-lynx178-tdep.c \
-	rs6000-nat.c \
 	rs6000-tdep.c \
 	rx-tdep.c \
 	s390-linux-nat.c \

gdb/configure.nat

@@ -103,7 +103,7 @@ case ${gdb_host} in
 		# Host: IBM PowerPC running AIX aix-thread.o is not
 		# listed in NATDEPFILES as it is pulled in by
 		# configure.
-		NATDEPFILES="${NATDEPFILES} rs6000-nat.o"
+		NATDEPFILES="${NATDEPFILES} rs6000-aix-nat.o"
 
 		# When compiled with cc, for debugging, this argument
 		# should be passed.  We have no idea who our current