[mips] Add support for calling convention CC_MipsO32_FP64, which is used when the
size of floating point registers is 64-bit.

Test case will be added when support for mfhc1 and mthc1 is added.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188847 91177308-0d34-0410-b5e6-96231b3b80d8