After two reports on IRC of issues building depends on an Apple M1
machine, it turns out that this option can't be used when targeting
arm-apple-darwin. For now, just use it for x86_64-apple-darwin.
```bash
Apple clang version 12.0.5 (clang-1205.0.22.9)
Target: x86_64-apple-darwin20.4.0
error: option 'cf-protection=return' cannot be specified on this target
error: option 'cf-protection=branch' cannot be specified on this target
2 errors generated.
```
3caedb4c03