[mips] Add assembler support for .set arch=x directive.

Summary:
This directive is similar to ".set mipsX".
It is used to change the CPU target of the assembler, enabling it to accept instructions for a specific CPU.

This patch only implements the r4000 CPU (which is treated internally as generic mips3) and the generic ISAs.

Contains work done by Matheus Almeida.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D4884