kbuild: Add generic hook for architectures to use before the final vmlinux link

On powerpc, we would like to be able to make a pass on vmlinux.o and
generate a new object file to be linked into vmlinux. Add a generic pass
in Makefile.vmlinux that architectures can use for this purpose.

Architectures need to select CONFIG_ARCH_WANTS_PRE_LINK_VMLINUX and must
provide arch/<arch>/tools/Makefile with .arch.vmlinux.o target, which
will be invoked prior to the final vmlinux link step.

Acked-by: Masahiro Yamada <[e-mail]>
Signed-off-by: Naveen N Rao <[e-mail]>
Signed-off-by: Hari Bathini <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

arch/Kconfig

···
 config ARCH_NEED_CMPXCHG_1_EMU
 	bool
 
+config ARCH_WANTS_PRE_LINK_VMLINUX
+	bool
+	help
+	  An architecture can select this if it provides arch/<arch>/tools/Makefile
+	  with .arch.vmlinux.o target to be linked into vmlinux.
+
 endmenu

scripts/Makefile.vmlinux

···
 vmlinux: .vmlinux.export.o
 endif
 
+ifdef CONFIG_ARCH_WANTS_PRE_LINK_VMLINUX
+vmlinux: arch/$(SRCARCH)/tools/vmlinux.arch.o
+
+arch/$(SRCARCH)/tools/vmlinux.arch.o: vmlinux.o FORCE
+	$(Q)$(MAKE) $(build)=arch/$(SRCARCH)/tools $@
+endif
+
 ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)
 
 # Final link of vmlinux with optional arch pass after final link

scripts/link-vmlinux.sh

···
 	${ld} ${ldflags} -o ${output}					\
 		${wl}--whole-archive ${objs} ${wl}--no-whole-archive	\
 		${wl}--start-group ${libs} ${wl}--end-group		\
-		${kallsymso} ${btf_vmlinux_bin_o} ${ldlibs}
+		${kallsymso} ${btf_vmlinux_bin_o} ${arch_vmlinux_o} ${ldlibs}
 }
 
 # generate .BTF typeinfo from DWARF debuginfo
···
 fi
 
 ${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init init/version-timestamp.o
+
+arch_vmlinux_o=
+if is_enabled CONFIG_ARCH_WANTS_PRE_LINK_VMLINUX; then
+	arch_vmlinux_o=arch/${SRCARCH}/tools/vmlinux.arch.o
+fi
 
 btf_vmlinux_bin_o=
 kallsymso=