target-mips: avoid tcg internal error in mfc0/dmfc0

Set t0 to 0 for unimplemented mfc0/dmfc0 instructions. This fixes a tcg
internal error while booting mips linux.

Noticed by Julia Longtin.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5723 c046a42c-6fe2-441c-8c8c-71466251a162