Assist building of gitaly

What does this Merge Request do and why?

⚠ This MR needs gitaly!4312 (closed) merged first.

To ensure gitaly can be built on all macOS architectures (x86_64 and arm64), the GDK will create <GDK_ROOT>/gitaly/config_generated.mak which currently sets the GIT_APPEND_BUILD_OPTIONS variable if on macOS.

Related: gitaly!4312 (closed)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.