project-lakitu: Fix sys-boot/shim build for ARM64

- Pass expected ARCH value expected by the shim Makefile
- Let makefile modify ARCH_LDFLAGS value passed from command-line

BUG=b/175740683
TEST=presubmit
RELEASE_NOTE=None

Change-Id: I2219c0f61b065c7caa03a16f8b973ea16686760d
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/12110
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Roy Yang <royyang@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
3 files changed