[llvm] [Mips] Add the missing judgment when processing function handleMFLOSlot (PR #121463)

[via llvm-commits]

yingopq wrote:

> I guess that there may be some other problem: why this patch can effects MIPSr2? I guess that the default port of FreeBSD is r2？

In file "clang/lib/Driver/ToolChains/Arch/Mips.cpp", have the following define.
```
// MIPS2 is the default for mips(el)?-unknown-freebsd.
  // MIPS3 is the default for mips64(el)?-unknown-freebsd.
  if (Triple.isOSFreeBSD()) {
    DefMips32CPU = "mips2";
    DefMips64CPU = "mips3";
  }
```

https://github.com/llvm/llvm-project/pull/121463