[M68k] Fix ODR violation in GISel code (#72797)

It prevents LLVM from being linked with LLD at least on Windows, with
errors like this:

```
  = note: ld.lld: error: duplicate symbol: vtable for llvm::FormalArgHandler
          >>> defined at librustc_llvm-a81737dd65a7c126.rlib(M68kCallLowering.cpp.obj)
          >>> defined at librustc_llvm-a81737dd65a7c126.rlib(PPCCallLowering.cpp.obj)
```

Binutils linker also complains about this, but only with warnings.

`FormalArgHandler` has a base class `M68kIncomingValueHandler` which
doesn't have a virtual method `markPhysRegUsed` like
`IncomingValueHandler`s for all other targets including PPC, so it
results in a conflict.
The simplest fix is to rename the `FormalArgHandler` structure (rather
than to add virtual methods for compatibility).

cc https://github.com/rust-lang/rust/issues/107668