MIPS: Fix asm constraints "f" and "r" for softfloat (#79116) This include 2 fixes: 1. Disallow 'f' for softfloat. 2. Allow 'r' for softfloat. Currently, 'f' is accpeted by clang, then LLVM meets an internal error. 'r' is rejected by LLVM by: couldn't allocate input reg for constraint 'r'. Fixes: #64241, #63632