Allow passing a custom os package to std

[MasterQ32]

While working on microzig, we got the following question:

How do we solve the startup problem? std/start.zig isn't made for custom OSes and forcing the user to type a pub const panic = microzig.panic; in each project is just boilerplate.

Therefore, i propose that zig can take a new command line argument --other-os-implementation which will provide a package definition os to std that can supplement the OS functions and startup code for any target that compiles for .os_tag = .other. This package should be able to have other packages as dependencies.

This allows us to write code with microzig that just looks like any other zig project:

const std = @import("std");

pub fn main() !void {
  std.log.info("Hello, World!\n", .{});
}

Related issues:

• Eliminate the POSIX API layer #6600

[sengir]

What about the semi-existing mechanism of root.os? Should this be deprecated in favor of your proposal or should root.os be expanded upon and standardized instead?

[marler8997]

Also related, made this issue when I was exploring GameboyAdvance development: #8508

[MasterQ32]

What about the semi-existing mechanism of root.os? Should this be deprecated in favor of your proposal or should root.os be expanded upon and standardized instead?

This behaviour would be deprecated, as root.os is a workaround for adding a posixy replacement layer. The problem is that not every OS works like this and i think we should go to some higher abstraction than POSIX