mips: Add nanoMIPS support to Valgrind 1/4

mips: Add nanoMIPS support to Valgrind 1/4

Necessary changes to support nanoMIPS on Linux.

Part 1/4 - VEX changes

Patch by Aleksandar Rikalo, Dimitrije Nikolic, Tamara Vlahovic and
Aleksandra Karadzic.

nanoMIPS architecture in brief

Designed for embedded devices, nanoMIPS is a variable lengths instruction
set architecture (ISA) offering high performance in substantially reduced
code size.

The nanoMIPS ISA combines recoded and new 16-, 32-, and 48-bit instructions
to achieve an ideal balance of performance and code density.
It incorporates all MIPS32 instructions and architecture modules including
MIPS DSP and MIPS MT, as well as new instructions for advanced code size
reduction.

nanoMIPS is supported in release 6 of the MIPS architecture. It is first
implemented in the new MIPS I7200 multi-threaded multi-core processor
series. Compiler support is included in the MIPS GNU-based development
tools.

Related KDE issue: #400872.