[mips] Add support for -modd-spreg/-mno-odd-spreg

Summary:
When -mno-odd-spreg is in effect, 32-bit floating point values are not
permitted in odd FPU registers. The option also prohibits 32-bit and 64-bit
floating point comparison results from being written to odd registers.

This option has three purposes:
* It allows support for certain MIPS implementations such as loongson-3a that
  do not allow the use of odd registers for single precision arithmetic.
* When using -mfpxx, -mno-odd-spreg is the default and this allows us to
  statically check that code is compliant with the O32 FPXX ABI since mtc1/mfc1
  instructions to/from odd registers are guaranteed not to appear for any
  reason. Once this has been established, the user can then re-enable
  -modd-spreg to regain the use of all 32 single-precision registers.
* When using -mfp64 and -mno-odd-spreg together, an O32 extension named
  O32 FP64A is used as the ABI. This is intended to provide almost all
  functionality of an FR=1 processor but can also be executed on a FR=0 core
  with the assistance of a hardware compatibility mode which emulates FR=0
  behaviour on an FR=1 processor.

* Added '.module oddspreg' and '.module nooddspreg' each of which update
  the .MIPS.abiflags section appropriately
* Moved setFpABI() call inside emitDirectiveModuleFP() so that the caller
  doesn't have to remember to do it.
* MipsABIFlags now calculates the flags1 and flags2 member on demand rather
  than trying to maintain them in the same format they will be emitted in.

There is one portion of the -mfp64 and -mno-odd-spreg combination that is not
implemented yet. Moves to/from odd-numbered double-precision registers must not
use mtc1. I will fix this in a follow-up.

Differential Revision: http://reviews.llvm.org/D4383