Various CI fixes

[hanna-kruppe]

Took a bunch of trial and error I mostly managed to preserve the existing test coverage, as I understood it. Notable exceptions:

• Ancient Rust versions no longer run tests on macOS runners because the ARM64 ones aren't a supported target on ancient Rust and the Intel macOS runners that still exist ran into some weird linker error I didn't know how to resolve.
• Some of the "weird targets" have been removed entirely (emscripten) or no longer have pre-built artifacts (mips-unknown-linux-musl). I haven't checked if there's another MIPS target that could be substituted, but I added s390x Linux so there's at least one big endian target in the mix.
• I removed the Solaris build because mio doesn't compile on it any more.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.93%. Comparing base (172283a) to head (2c3b402).
⚠️ Report is 43 commits behind head on master.
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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #193       +/-   ##
===========================================
+ Coverage   60.20%   72.93%   +12.73%     
===========================================
  Files          16       17        +1     
  Lines         784     1090      +306     
  Branches      117        0      -117     
===========================================
+ Hits          472      795      +323     
- Misses        241      295       +54     
+ Partials       71        0       -71     

Files with missing lines	Coverage Δ
serial_test/src/lib.rs	100.00% <100.00%> (ø)
signal-hook-registry/src/lib.rs	85.43% <ø> (+8.03%)	⬆️
signal-hook-tokio/src/lib.rs	93.93% <ø> (-6.07%)	⬇️
src/iterator/mod.rs	77.04% <ø> (ø)
src/low_level/mod.rs	50.00% <ø> (+10.00%)	⬆️

... and 13 files with indirect coverage changes

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[vorner]

This is a big help, thank you :-)

Can you have a look at the windows failure, though? It seems the failure is legitimate. Or, I assume this was there for some time already and your changes only uncovered the bug?

[hanna-kruppe]

I didn't find a nice, maintainable way to have CI omit only the extended-siginfo feature only when testing on Windows. So instead I've gated the functionality of the feature behind cfg(not(windows)), mirroring low_level::pipe. In other words, the feature can be enabled on Windows but it won't do anything, other than enabling an unused cc dependency.

After some more whack-a-mole with windows-specific warnings and errors, the only remaining failure is the coverage workflow, that seems to be due to repository configuration (it worked on my fork), so I can't resolve that.