Mangle C main on Wasm

[QuantumSegfault]

Mangles extern(C) main as __main_void or __main_argc_argv on Wasm targets, as done by Clang, and expected by the libcs for Emscripten and WASI.

Split from ldc-developers/ldc#5158
Dependent on remainder of said PR to introduce gen.llvmhelpers.isTargetWasm.

[QuantumSegfault]

Requesting review

@thewilsonator

[thewilsonator]

cc @ibuclaw for whatever the GCC logic should be

[dkorpel]

DMD has no WASM target, so this changelog entry shouldn't be here

[QuantumSegfault]

So even though the change was upstreamed, it shouldn't be logged?

What about #23293 and #23294?

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Either way, this particular one has already been merged. Do you need me to open a PR to delete it?

[dkorpel]

It can be logged in LDC's release notes, but the changelog in this repo gets published with dmd releases so users are aware of what changed when upgrading dmd.

Newly reserved version WASI identifiers affect dmd because version = WASIp1 is something dmd now rejects. core.stdc updates for WASM could be relevant if anyone used dmd's bundled druntime sources with LDC, but that sounds very niche so I don't think it's worth mentioning. New mangling of WASM main is irrelevant/confusing for dmd users because it is incapable of generating any WASM code. I would delete it indeed.