This requires manual defines before C99 which may not have stdint.h.

Also have update-zig1 leave a copy of lib/zig.h in stage1/zig.h, which
allows lib/zig.h to be updated without needing to update zig1.wasm.
Note that since the object already existed with the exact same contents,
this completely avoids repo bloat due to zig.h changes.