add zig build system for luv

[truemedian]

This is the first step in allowing simple cross compilation for luv and luvi via zig.

This code needs to be compared against the respective Makefiles and CMakeLists before this can be merged.

This is marked as a draft PR as some features (specifically tests) need to be able to pass successfully on all the supported platforms before this can be any semblance of a good way to build luv.

At the moment: tests/test-signal's SIGINT test needed to be modified to run os.exit(0), this needs to be checked, ~~the built luajit executable is being flagged by UBSan~~

[squeek502]

the built luajit executable is being flagged by UBSan

I assume this is being worked around via disable_sanitize_c = true? Or is it still triggering UBSan checks with that?

Seems like it would be nice to try adding some more architectures to CI with this (aarch64 definitely, probably via cross-compilation and then run with qemu?)

[truemedian]

I assume this is being worked around via disable_sanitize_c = true?

It should be, but yes, its still emitting ub1 instructions and I didn't take the time to figure out why.

Seems like it would be nice to try adding some more architectures to CI

I'll play around a bit with making github actions workflows for some of the major ones (linux64, macos64, win64/32) once I can get it all working correctly and passing tests.

[truemedian]

I figured out why luajit was being ubsan'd (the library itself needed no-sanitize-c), however, something in libc's atexit is causing a segfault, and I can't figure out why that would be happening. It seems to be happening with both luajit and PUC, so its likely an issue with building libuv.

Edit: it appears that luv registers an atexit() handler, which is problematic as luv is loaded as a dynamic library, meaning that in a normally exiting program, the lua state will be closed (and luv unloaded) before atexit() handlers are called. This leads to a segfault.