[stable] druntime: Add Emscripten support in core.sys.posix.signal

[kinke]

Based on https://github.com/emscripten-core/emscripten/blob/main/system/lib/libc/musl/arch/emscripten/bits/signal.h. This makes core.stdc.stdio importable again for Emscripten (tested by LDC), which regressed with v2.113.

[dlang-bot]

Thanks for your pull request and interest in making D better, @kinke! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

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dub run digger -- build "stable + dmd#22901"