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

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Co-authored-by: Fangrui Song <i@maskray.me>
(cherry picked from commit c88beb41)