[github-actions] Add multi-arch support

Extended the kernel build and test workflow to support multiple architectures.
The workflow now builds and tests kernels for both x86_64 and aarch64 platforms in parallel.

Important changes :-

- Added new kernel-build-and-test-multiarch.yml workflow
- Kept existing kernel-build-and-test-x86_64.yml for x86_64-only builds
- Added dynamic matrix generation based on architecture input parameter
- Separate runners for each architecture (kernel-build for x86_64, kernel-build-arm64 for aarch64)
- Per-architecture artifact uploads and comparison results
- Architecture-specific kselftest baseline tracking and regression detection
- PR creation includes results from both architectures with separate status sections
- Uses new create-pr-body-multiarch.sh script for PR generation

Signed-off-by: Shreeya Patel <spatel@ciq.com>

Author: shreeya-patel98