WIP: Improve Emscripten support

[kinke]

Based on WASI.

[kinke]

@QuantumSegfault: This seems like a viable approach - basing Emscripten on your WASI work, after reading https://v8.dev/blog/emscripten-standalone-wasm and noting that they apparently try to use WASI APIs as much as possible, even in non-standalone-wasm mode.

To get the druntime test runners to link, I only had to avoid a single undefined _CLOCK_MONOTONIC symbol. And a single module fails its unittests (core.sync.event): https://github.com/ldc-developers/ldc/actions/runs/31392028311/job/93465932716?pr=5259

[QuantumSegfault]

I'm not sure this makes sense to do like this.

Emscripten use a more vanilla Musl, without all of the differences to the headers WASI-libc made. They opted to stub out functions, rather than remove them, and otherwise use the unmodified types, constants, etc. (other than where architecture specific differences are concerned). A lot of the CRuntime_WASI blocks, particularly in Phobos, may be unnecessary.

WASI is also iffy, as that's an internal implementation detail. Emscripten also provides fuller emulation of things like threads, that we disable tests for on WASI. Also they provide their own filesystem, which isn't subject to all the quirks of WASI (e.g. the inability to have absolute symlinks; though that's largely a Wasmtime limitation).