kbuild: improve linker compatibility with lib-ksyms.o build

lib-ksyms.o is created by linking an empty input file with a linker
script containing the interesting bits. Currently the empty input file
is an archive containing nothing, however this causes the gold linker
to segfault.

I have opened a bug against gold

  https://sourceware.org/bugzilla/show_bug.cgi?id=20767

However this can be worked around by assembling an empty file to link
with instead. The resulting lib-ksyms.o is slightly larger (seemingly
due to empty .text, .data, .bss setions added), but final linked
output should not be changed.
Reported-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NNicholas Piggin <npiggin@gmail.com>
Signed-off-by: NMichal Marek <mmarek@suse.com>

scripts/Makefile.build

@@ -424,7 +424,7 @@ quiet_cmd_export_list = EXPORTS $@
 cmd_export_list = $(OBJDUMP) -h $< | \
 	sed -ne '/___ksymtab/{s/.*+/$(ref_prefix)/;s/ .*/)/;p}' >$(ksyms-lds);\
 	rm -f $(dummy-object);\
-	$(AR) rcs$(KBUILD_ARFLAGS) $(dummy-object);\
+	echo | $(CC) $(a_flags) -c -o $(dummy-object) -x assembler -;\
 	$(LD) $(ld_flags) -r -o $@ -T $(ksyms-lds) $(dummy-object);\
 	rm $(dummy-object) $(ksyms-lds)