nds32: fix building failed if using elf toolchain.

OUTPUT_FORMAT is not necessary here and the elf toolchain doesn't
support these formats. Since kernel should be built pass with elf
or Linux toolchain. This can be removed from vdso.ld.S

These are the built failed messages.
  VDSOL   arch/nds32/kernel/vdso/vdso.so.dbg
  /home/users/greentime/tmp/nds32le-elf-newlib-v3-upstream-b224/bin/../lib/gcc/nds32le-elf/8.0.1/../../../../nds32le-elf/bin/ld:
  target elf32-nds32le-linux not found
  collect2: error: ld returned 1 exit status
  make[1]: *** [arch/nds32/kernel/vdso/vdso.so.dbg] Error 1
  make: *** [vdso_prepare] Error 2

Signed-off-by: Greentime Hu <greentime@andestech.com>