target-mips: add SIGRIE instruction

Add SIGRIE (Signal Reserved Instruction Exception) for both MIPS and
microMIPS.
The instruction allows to use the 16-bit code field for software use.
This instruction is introduced by and required as of Release 6.

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>