macho: use start.zig for macOS entrypoint

This effectively allows us to compile

```zig
pub fn main() void {}
```

which then calls into `std.start`.

Changes required to make this happen:
* handle signed int to immediate in x86_64 and aarch64 codegen
* ensure that on arm64 macOS, `.x19` is a caller-preserved register -
  I'm not sure about that one at all and would like to brainstorm it
  with anyone interested and especially Joachim.
* finally, fix a bug in the linker - mark new got entry as dirty upon
  atom growth.

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