MIPS: uasm: Add s3s1s2 instruction builder

It will be used later on by the SLT instruction.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7119/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>