avoid depending on child process execution when the target OS does not support it

[andrewrk]

There are a few places in the Zig compiler where we invoke a child process, but none of them are strictly necessary. This issue is to detect when the OS does not support child process execution and produce a build of the compiler that does not attempt to use child processes.

For now the detection logic can be simply:

const can_spawn = switch (builtin.os.tag) {
    .wasi => false,
    else => true,
};

Here is a complete list of everywhere Zig might want to spawn a child process:

• zig test - instead print a message about not being able to spawn child processes and exit success.
• zig run - instead print "unable to spawn child processes" and exit failure.
• for clang we can invoke clang_main directly instead of as a child process
• for lld we can invoke the lld library functions instead of as child process
• same thing for zig ar, zig dlltool, etc.
• for detecting native libc, just return error.LibcNotDetected or whatever the appropriate error is.

This is a prerequisite for #10716.

[marler8997]

I'd be curious to see how significant the performance cost is for sub processes on platforms that support it. With this in place we could easily find out!

[zigazeljko]

Doesn't zig build also spawn child processes?

[fogti]

Why should zig test exit with success?

[marler8997]

Doesn't zig build also spawn child processes?

Oh yes. To compile anything zig build will spawn a subprocess like zig build-exe ....

[matu3ba]

This should probably be first implemented before #1356 with potentially panicking subprocess being started from the subprocess that supervises test results.

[andrewrk]

Landed in 2113538