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
9eb15535.

Commit 91f205ac 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>