Issue Description:

As pointed out by @weissi in this GitHub comment, the Android logic in stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift has different semantics to posix_spawn. posix_spawn will give you a proper errno value if the spawn failed, whereas the Android logic just exits the child process. I.e. spawning ["/bin/zsh", "-c", "exit 126;"] will be indistinguishable for the caller.

@weissi proposes opening a separate pipe, called child2parent or so which will be fcntl'd to FD_CLOEXEC. That means if execve succeeds, it will be closed automatically and that means the parent will read EOF. If execve fails, we just write execve's errno to that pipe so the parent can read it and learns what went wrong in the child. So the parent waits until it can read, if it reads EOF (read returns 0) it knows, the child spawned successfully, if it reads anything else, it knows that this is execve's errno value and can return that to the caller.

@weissi even provides some pseudo-C code to demonstrate:

    [...]
    } else if (pid == 0) {
        int err = fcntl(child2parent[1], F_SETFD, FD_CLOEXEC); /* This fd will be closed on exec */
        int errno_save = errno;
        if (err) {
           preconditionFailure("fcntl failed, errno: %d", errno_save);
           abort();
        }
        execve(...);
        errno_save = errno;
        /* execve returned, this is an error that we need to communicate back to the parent */
        ssize_t suc_write = write(child2parent[1], &errno_save, sizeof(typeof(errno)));
        errno_save = errno;
        if (suc_write > 0 && suc_write < sizeof(typeof(errno)));
            /* implement write retry ;) */
        } else if (suc_write == 0) {
            /* preconditionFailure("EOF?!"); */
            abort();
        } else if (suc_write < 0) {
            preconditionFailure("write failed, errno %d", errno_save);
            abort();
        }
        close(child2parent[1]);
    }
    [...]