[mips] Add calling convention tests covering O32, N32, and N64.

Summary:
I had difficulty finding tests for the N32 and N64 ABI so I've added a
collection of calling convention tests based on the document MIPS ABIs
Described (MD00305), the MIPSpro N32 Handbook, and the SYSV ABI. Where the
documents/implementations disagree, I've used GCC to resolve the conflict.

A few interesting details:
* For N32, LLVM uses 64-bit pointers when saving $ra despite pointers being
  32-bit. I've yet to find a supporting statement in the ABI documentation but
  the current behaviour matches GCC.

* For O32, the non-variable portion of a varargs argument list is also subject
  to the rule that floating-point is passed via GPR's (on N32/N64 only the
  variable portion is subject to this rule). This agrees with GCC's behaviour
  and the SYSV ABI but contradicts part of the MIPSpro N32 Handbook which talks about O32's behaviour.

* The N32 implementation has the wrong callee-saved register list.
  (I already have a fix for this but will commit it as a follow-up).

I've left RUN-TODO lines in for O32 on MIPS64. I don't plan to support this case
for now but we should revisit it.

Reviewers: matheusalmeida, vmedic

Reviewed By: matheusalmeida

Differential Revision: http://reviews.llvm.org/D3339