[S390] vdso: add missing vdso_install target

Add missing vdso_install target to install the unstripped vdso images
into $(MODLIB)/vdso/.  These files are helpful when containing
additional debugging information.
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Makefile

@@ -116,6 +116,12 @@ image bzImage: vmlinux
 zfcpdump:
 	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
 
+vdso_install:
+ifeq ($(CONFIG_64BIT),y)
+	$(Q)$(MAKE) $(build)=arch/$(ARCH)/kernel/vdso64 $@
+endif
+	$(Q)$(MAKE) $(build)=arch/$(ARCH)/kernel/vdso32 $@
+
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)