Auto merge of #120348 - bjorn3:per_target_backend_selection, r=albert…

…larsan68

Support configuring the set of codegen backends to build per host triple

This allows building the compiler itself with one backend while using another backend at runtime. For example this allows compiling rustc to wasm using LLVM, while using Cranelift at runtime to produce actual code. Cranelift can't compile to wasm, but is perfectly capable of running on wasm. LLVM can compile to wasm, but can't run on wasm. [^1]

[^1]: The prototype of this still requires a couple of other patches.