[Mips] Remove "single" from the list of valid MIPS float ABI names. Add
two new options –msingle-float and –mdouble-float. These options can be
used simultaneously with float ABI selection options (-mfloat-abi,
-mhard-float, -msoft-float). They mark whether a floating-point
coprocessor supports double-precision operations.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179481 91177308-0d34-0410-b5e6-96231b3b80d8