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