scripts/gdb: refactor rules for symlink creation

gdb-scripts is not a real object, but (ab)used like a phony target.

Rewrite the code in a more Kbuild-ish way. Add symlinks to extra-y
and use if_changed.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

scripts/gdb/linux/Makefile

 # SPDX-License-Identifier: GPL-2.0
-always := gdb-scripts
 
-SRCTREE := $(abspath $(srctree))
-
-$(obj)/gdb-scripts:
 ifneq ($(KBUILD_SRC),)
-	$(Q)ln -fsn $(SRCTREE)/$(obj)/*.py $(objtree)/$(obj)
+
+symlinks := $(patsubst $(srctree)/$(src)/%,%,$(wildcard $(srctree)/$(src)/*.py))
+
+quiet_cmd_symlink = SYMLINK $@
+      cmd_symlink = ln -fsn $(patsubst $(obj)/%,$(abspath $(srctree))/$(src)/%,$@) $@
+
+extra-y += $(symlinks)
+$(addprefix $(obj)/, $(symlinks)): FORCE
+	$(call if_changed,symlink)
+
 endif
-	@:
 
 quiet_cmd_gen_constants_py = GEN     $@
       cmd_gen_constants_py = \
@@ -18,4 +22,4 @@ extra-y += constants.py
 $(obj)/constants.py: $(src)/constants.py.in FORCE
 	$(call if_changed_dep,gen_constants_py)
 
-clean-files := *.pyc *.pyo $(if $(KBUILD_SRC),*.py)
+clean-files := *.pyc *.pyo