Build instructions for wasm branch.

1. Create config.toml:

[build]
submodules = false
target = ["x86_64-unknown-linux-gnu", "wasm32-unknown-emscripten"]

[rust]
debug-assertions = true

[llvm]
assertions = true
targets = "X86"
experimental-targets = "WebAssembly"

2. Checkout the wasm3 branch both in rust and in src/llvm submodule. (I haven't committed LLVM changes into rust repo for ease of development, so you'll have to do this manually).
3. git status, then run git submodule update <sumbodule> for all submodules other than src/llvm.
4. x.py build --stage=1, as usual.