[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.

llvm-svn: 179481