PAC missing from GNU Notes

While it doesn't impact PAC functionality, it is nice for auditing.
```
readelf -n ./aarch64-unknown-linux-gnu/.libs/libffi.so

Displaying notes found in: .note.gnu.property
  Owner                Data size 	Description
  GNU                  0x00000010	NT_GNU_PROPERTY_TYPE_0
      Properties: AArch64 feature: BTI
```

I have a patch incoming.
