ci(gitlab): refactor GitLab pipeline script to add windows-x86_64 build and test stages
- remove matrix based job definitions in favor of explicit OS-specific jobs
- introduce separate runner presets for linux_x86_64 and windows_x86_64
- create build/test templates for reuse across platforms
- split build and test stages into per-OS jobs with explicit names
- simplify cache keys to include pipeline ID and OS
Signed-off-by: Deeptanshu Sekhri deeptanshu.sekhri@arm.com