Support sigaction (for LLVM)

[theoparis]

After I tried updating this PR to LLVM 18 I ran into an issue struct sigaction being an incomplete type...

This would also depend on WebAssembly/exception-handling#179, because jmp_buf seems to be missing. Alternatively it is possible to disable any sigaction types and setjmp/jmp_buf from being referenced in LLVM but that's not a permanent solution and it won't get merged upstream.

[whitequark]

See this RFC where I propose to have it be merged upstream.

[abrown]

In talking to @alexcrichton, @sbc100, @sunfishcode: signals are a bigger topic which we think should be discussed at the core WebAssembly level (see WebAssembly/design#1446).