target-mips: Correct the writes to Status and Cause registers via gdbstub

target-mips: Correct the writes to Status and Cause registers via gdbstub

Make writes to CP0.Status and CP0.Cause have the same effect as
executing corresponding MTC0 instructions would in Kernel Mode.  Also
ignore writes in the user emulation mode.

Currently for requests from the GDB stub we write all the bits across
both registers, ignoring any read-only locations, and do not synchronise
the environment to evaluate side effects.  We also write these registers
in the user emulation mode even though a real kernel presents them as
read only.

Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>