Adds pkgsCross.wasm32 and pkgsCross.wasm64. Use it to build Nixpkgs
with a WebAssembly toolchain.
stdenv/cross: use static overlay on isWasm
isWasm doesn’t make sense dynamically linked.
this adds libc++ to the LLVM cross, giving us access to the full
Nixpkgs set. This requires 4 stages of wrapped compilers:
- Clang with no libraries
- Clang with just compiler-rt
- Clang with Libc, and compiler-rt
- Clang with Libc++, Libc, and compiler-rt
