Use clang + wasi-sdk instead of emcc to compile to WASM

[paulcadman]

clang supports wasm{32,64}-unknown-unknown and wasm{32,64}-unknown-wasi compilation targets. And the WebAssembly project provides a C toolchain that provides a libc for things like malloc, free, io etc..

For our use-case this seems like a better approach than emscripten (which is a project more focused on generating WASM/JS for browser runtime).

The goal of this issue is to get the minic tests running in CI with clang compiler.

Setup WASM cross compilation toolchain

wasi-sdk publishes precompiled releases of the wasm toolchain. The libclang_rt.builtins-wasm32-wasi-15.0.tar.gz archive should be extracted in the clang installation root (e.g /usr/lib/clang/14 on Ubuntu). And the wasi-sysroot-15.0.tar.gz archive can be extracted anywhere locally.

Clang can then be used as follows:

clang-14 -nodefaultlibs -lc --target=wasm32-wasi --sysroot /home/ubuntu/wasi-sysroot hello.c -o hello.wasm

Where /home/ubuntu/wasi-sysroot is the path where the sysroot archive was extracted to.

-nodefaultlibs -lc is important here because (on ubuntu, perhaps other linux systems) clang passes -lc -lgcc to the linker by default (and gcc lib is not available in the wasm sysroot).