Issue: https://github.com/ziglang/zig/issues/1117

The macOS stage1 Zig compiler should look in Zig's real absolute path
for the Zig stdlib, but os_self_exe_path looks in its path as returned
by _NSGetExecutablePath, which may be a symlink.  This means that a
symlinked Zig cannot find the Zig stdlib.

This patch fixes the issue by resolving the _NSGetExecutablePath result
to the real path using realpath() before copying the result to the
output path.