[MIPS] Cleanup CP0 Status initialization

Add setup_c0_status from Linux. For the moment we disable interrupts, set
CU0, mark the kernel mode, and clear ERL and EXL. This is good enough for
reset-time configuration and will work well across most processors.

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>