Initial WasmLinkerTool implementation and supporting cleanup.

[ScottTodd]

Progress on #4019. I can generate .wasm files using iree-translate -iree-hal-target-backends=dylib-llvm-aot -iree-llvm-target-triple=wasm32-unknown-unknown -iree-llvm-keep-linker-artifacts and can inspect the .wasm artifacts using wasm-decompile and the other tools from https://github.com/WebAssembly/wabt. I haven't yet been able to actually call a function from a generated .wasm file and inspect its output though, so we could be missing some configuration.

This PR also includes some supporting changes in the other linker tools, mostly playing cross-platform development whack-a-mole until the code is robust enough. As one side effect, I can now use UnixLinkerTool from Windows, by passing -iree-llvm-target-triple=x86_64-linux-gnu, which discovers C:\Program Files\LLVM\bin\ld.lld on my system, from having that bin\ directory on my PATH.

[benvanik]

Nice! Using the unix linker tool on windows is a good improvement as that's a valid scenario, and if we have our own elf loader then we'd be using elfs for that and could build them on windows. May be worth renaming WindowsLinkerTool to MSVCLinkerTool.