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