[mips] Do not emit '.module [no]oddspreg' unless we really need to.

We now emit this directive when we need to contradict the default value (e.g.
-mno-odd-spreg is given) or an option changed the default value (e.g. -mfpxx
is given).

This restores support for the currently available head of binutils. However,
at this point binutils 2.24 is still not sufficient since it does not support
'.module fp=...'.