kernel: add workaround to rebuild vdso-o32.so.dbg

Building for octeon fails with

  'arch/mips/vdso/vdso-o32.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
9eb155353a5f5137ec85a5b5b0287af63c544710.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49345 3c298f89-4303-0410-b956-a3cf2f4a3e73