Add support for more LLVM targets

Motivation

LLVM supports far more targets than the native code generator (NGC) available in GHC [1].

If GHC could be extended to provide LLVM backend support for LLVM targets such as M68k, MIPS, PowerPC and SPARC, it would for a faster compiler on these targets since LLVM-IR is more efficient than intermediate C code.

If LLVM support was available for 32-bit PowerPC, for example, it could easily be used to work around #23969 (closed).

Proposal

Enable LLVM target support for M68k, MIPS, PowerPC and SPARC.

[1] https://github.com/llvm/llvm-project/tree/main/llvm/lib/Target