tools/debugger/gdbsx: Fix and cleanup makefiles

tools/debugger/gdbsx: Fix and cleanup makefiles

gdbsx/:
  - Make use of subdir facility for the "clean" target.
  - No need to remove the *.a, they aren't in this dir.
  - Avoid calling "distclean" in subdirs as "distclean" targets do only
    call "clean", and the "clean" also runs "clean" in subdirs.
  - Avoid the need to make "gx_all.a" and "xg_all.a" in the "all"
    recipe by forcing make to check for update of "xg/xg_all.a" and
    "gx/gx_all.a" by having "FORCE" as prerequisite. Now, when making
    "gdbsx", make will recurse even when both *.a already exist.
  - List target in $(TARGETS).

gdbsx/*/:
  - Fix dependency on *.h.
  - Remove some dead code.
  - List targets in $(TARGETS).
  - Remove "build" target.
  - Cleanup "clean" targets.
  - remove comments about the choice of "ar" instead of "ld"
  - Use "$(AR)" instead of plain "ar".

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>