[mips] Rename FeatureMadd4 to FeatureNoMadd4. NFC

`FeatureMadd4` is used to disable `madd4`, and the corresponding feature
option is `(+-)nomadd4`. Renaming to the `FeatureNoMadd4` makes its
purpose clear.

Patch by YunQiang Su.

Differential Revision: https://reviews.llvm.org/D83780