[mips] Add assembler support for '.set mipsX'.

Summary:
This patch also fixes an issue with the way the Mips assembler enables/disables architecture
features. Before this patch, the assembler never disabled feature bits. For example,
.set mips64
.set mips32r2

would result in the 'OR' of mips64 with mips32r2 feature bits which isn't right.
Unfortunately this isn't trivial to fix because there's not an easy way to clear
feature bits as the algorithm in MCSubtargetInfo (ToggleFeature) only clears the bits
that imply the feature being cleared and not the implied bits by the feature (there's a
better explanation to the code I added).

Patch by Matheus Almeida and updated by Toma Tabacu

Reviewers: vmedic, matheusalmeida, dsanders

Reviewed By: dsanders

Subscribers: tomatabacu, llvm-commits

Differential Revision: http://reviews.llvm.org/D4123