This change enables building arbitrary cross-compilers for non-x64 hosts. Right now, only osx-arm64->browser-wasm and linux-arm64->browser-wasm are actually enabled, as runtime bugs prevent us from building any arm64->x86 or arm64->x64 cross-compilers. Getting this framework in will unblock workload work related to "native" wasm on M1 Mac.

== BREAKING CHANGE FOR DEVELOPERS ==
MonoAOTLLVMDir no longer exists.

Old:
`/p:MonoLLVMDir=/path/to/llvm` specifies the directory with LLVM binaries compiled for `$(TargetArchitecture)`, for compiling the Mono JIT on that target architecture
`/p:MonoAOTLLVMDir=/path/to/llvm` specifies the directory with LLVM binaries compiled for `$(BuildArchitecture)`, for compiling the mono-aot-cross executable
```

New:
`/p:MonoLLVMDir=/path/to/llvm` specifies a parent-level directory with LLVM binaries compiled for relevant architectures - e.g. `$(MonoLLVMDir)\x64` contains LLVM built for `x64`, current OS. `$(MonoLLVMDir)\arm64 contains LLVM built for arm64`, current OS.

You do not need to specify `/p:MonoLLVMDir=` manually, unless you are a developer who may be using custom, non-nupkg LLVM.

This change is necessary to stop hardcoding the assumptions that a) `$(BuildArchitecture)` is always `x64` and b) AOT binaries should be compiled for $(BuildArchitecture)

Closes: #74175