zig build: run steps in parallel

[andrewrk]

Finally, after this PR, when you run zig build, it will actually use all your cores to do stuff in parallel.

After fixing any issues that crop up in zig's own build script, this should reduce the CI time, as well as make it more palatable to use zig build for local testing. For example, zig build test-translate-c in this branch took 0.8s for me vs 12s for master.

Follow-Up Issues:

• make sure the .pdb file is not regressed
• introduce std.io.poll #14744
• is the call to make() in CompileStep is problematic?
• eliminate calls to process.exit(1) in make() implementations, instead set the stderr of step result. this way we don't end up with dead, useless threads.
• avoid printing to stderr in make() implementations. capture it, and set it in step.result if the step fails. print it later after the step graph has been executed.
• actually it should print the stderr directly after it fails, just make sure to lock the stderr mutex and don't interrupt the progress bar output from the build runner
• give build_runner its own progress bar
• add a -j argument to override the default thread count
• zig's build script appears to have several issues when parallelism is introduced
• move the test-cases back over to the build system to take advantage of running things at the same time
• build: suggestion to make parallel build output more colorblind friendly #14937
• accept a seed option to the build runner and shuffle the dependency order using it #14940
• semantic error information communicated to the build runner
• progress information communicated to the build runner
• introduce build_runner.phase enum {configure, make} and assert the phase in many functions #14941
• handle panics during make() gracefully #14942
• make uninstall step based on inspecting install steps, rather than with "push installed file" mechanism #14943
• extract WriteSourceFileStep out from WriteFileStep #14944
• unify the RunStep checks logic with CheckFileStep code and also with std.testing #14945
• std.Progress: instead of one line of terminal progress, make it check the terminal width and use multiple lines #14946
• rename std.Build.FooStep to std.Build.Step.Foo #14947
• CI on Windows timing out sometimes #14948
• RunStep: learn target information from the binary on disk rather than knowledge of the target ahead of time #14949
• remove std.log usage from std.Build.Cache in favor of proper error reporting #14950
• remove CompileStep.output_dir from the build system API #14951
• miscompilation regarding pointer to optional slice #14952
• build runner: actually limit the peak RSS rather than enforcing it post-hoc #14953
• extract struct fields from std.Build which should go into a struct shared by all Build instances #14954
• support G, M, GiB, MiB, GB, MB, etc. shorthands in --maxrss arg #14955
• add Windows support for detecting maxrss of child process #14956
• add Darwin support for detecting maxrss of child process #14957
• warning(module): unexpected EOF reading cached ZIR for crypto/bcrypt.zig
• add .h file generation tests back #14958
• make the cwd of RunStep take a Cache.Directory #14959
• add addCheckFile method to FileSource #14960
• collect error return traces from errors other than error.MakeFailed #14961
• ld.lld: warning: Linking two modules of different target triples ... arm-unknown-linux-musleabihf vs armv6kz-unknown-linux-musleabihf #14962
• static libraries can be picked up instead of system libraries when -l is used #14963
• port uuid link test over to the new build system API #14964
• add a skip reason to show up in the build runner summary #14965
• add the incremental compilation tests back
• instead of embedding the compiler in the check-cases process, spawn the compiler as a sub-process and communicate the updates using the compiler protocol #14966
• eliminate warnings from the linker #14967
• unit tests that write files to cwd have racy failures #14968
• eliminate std.Build.CheckObjectStep.runAndCompare #14969
• test-cases: make it not link libc in the case of backend llvm
• custom test runner results in garbage printed to stderr
• set ZIG_DEBUG_COLOR depending on build runner ttyconf
• rename --verbose to --verbose-spawn and limit the scope of what it does #14970
• take away EmitOption.emit_to option and make it give a FileSource #14971
• communicate the error when a test fails in release mode #14972
• build: specify a step option to avoid being run in parallel #14934