[mips] Make it impossible to have UnknownABI in CodeGen and Integrated Assembler.

Summary:
This removes the need to coerce UnknownABI to the default ABI (O32 for
MIPS32, N64 for MIPS64 [*]) in both MipsSubtarget and MipsAsmParser.

Clang has been updated to disable both possible default ABI's before enabling
the ABI it intends to use.

[*] N64 being the default for MIPS64 is not actually correct.
    However N32 is not fully implemented/tested yet.

Depends on: D2830

Reviewers: jacksprat, matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D2832
Differential Revision: http://llvm-reviews.chandlerc.com/D2846