[mips] Do not emit '.module fp=...' unless we really need to.

We now emit this value when we need to contradict the default value. This
restores support for binutils 2.24.

When a suitable binutils has been released we can resume unconditionally
emitting .module directives. This is preferable to omitting the .module
directives since the .module directives protect against, for example,
accidentally assembling FP32 code with -mfp64 and producing an unusuable object.