kbuild: thin archives for multi-y targets

THIN_ARCHIVES builds archives for built-in.o targets, have it build
multi-y targets as archives as well.

This saves another ~15% of the size of intermediate artifacts in the
build tree. After this patch, the linker is only used in final link,
and special cases like vdsos.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.com>

scripts/Makefile.build

@@ -448,11 +448,18 @@ $($(subst $(obj)/,,$(@:.o=-objs)))    \
 $($(subst $(obj)/,,$(@:.o=-y)))       \
 $($(subst $(obj)/,,$(@:.o=-m)))), $^)
 
-quiet_cmd_link_multi-y = LD      $@
-cmd_link_multi-y = $(LD) $(ld_flags) -r -o $@ $(link_multi_deps) $(cmd_secanalysis)
+cmd_link_multi-link = $(LD) $(ld_flags) -r -o $@ $(link_multi_deps) $(cmd_secanalysis)
+
+ifdef CONFIG_THIN_ARCHIVES
+  quiet_cmd_link_multi-y = AR      $@
+  cmd_link_multi-y = rm -f $@; $(AR) rcST$(KBUILD_ARFLAGS) $@ $(link_multi_deps)
+else
+  quiet_cmd_link_multi-y = LD      $@
+  cmd_link_multi-y = $(cmd_link_multi-link)
+endif
 
 quiet_cmd_link_multi-m = LD [M]  $@
-cmd_link_multi-m = $(cmd_link_multi-y)
+cmd_link_multi-m = $(cmd_link_multi-link)
 
 $(multi-used-y): FORCE
 	$(call if_changed,link_multi-y)