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>