target-mips: add privilege level check to several Cop0 instructions

The MIPS Architecture Verification Programs (AVPs) check privileged
instructions for the required privilege level.  These changes are needed
to pass the AVP suite.

Signed-off-by: Eric Johnson <ericj@mips.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
