kernel: add workaround to rebuild vdso-n32.so.dbg too

Building for octeon fails with

  'arch/mips/vdso/vdso-n32.so.dbg' already contains a '.MIPS.abiflags'
  section

if the file already exists from a prior build.

Use the same workaround as the one for vdso.so.dbg committed in
9eb155353a.

Commit 91f205acaf extended the workaround
to cover vdso-o32.so.dbg but missed the vdso-n32.so.dbg which is added
now by this change.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/generic/patches-4.4/207-mips-vdso-dbg-rebuild-after-genvdso.patch

@@ -18,3 +18,12 @@
  	$(call if_changed,vdsold)
  
  $(obj)/vdso-o32-image.c: VDSO_NAME := o32
+@@ -145,7 +145,7 @@ $(obj)/vdso-n32.lds: KBUILD_CPPFLAGS :=
+ $(obj)/vdso-n32.lds: $(src)/vdso.lds.S FORCE
+ 	$(call if_changed_dep,cpp_lds_S)
+ 
+-$(obj)/vdso-n32.so.dbg: $(obj)/vdso-n32.lds $(obj-vdso-n32) FORCE
++$(obj)/vdso-n32.so.dbg: $(obj)/vdso-n32.lds $(obj-vdso-n32) $(obj)/genvdso FORCE
+ 	$(call if_changed,vdsold)
+ 
+ $(obj)/vdso-n32-image.c: VDSO_NAME := n32