Implement various microMIPS MT ASE helpers accroading to:

MIPS® Architecture for Programmers
Volume IV-f: The MIPS® MT Module for the microMIPS32™ Architecture

Fixes build error:
{standard input}:2616: Error: branch to a symbol in another ISA mode

This make MT ASE available on microMIPS as well.

Boot tested on M5150 with microMIPS enabled on M5150.
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>