fix: normalize kernel output dirs from x86_64 to amd64

[ValentaTomas]

The infra orchestrator's TargetArch() uses Go's runtime.GOARCH convention (amd64) but kernels were stored under x86_64/, causing gcsfuse stat misses and a ~15-40ms penalty per sandbox create on GCE nodes.

Map x86_64 -> amd64 in build output so GCS paths match what the orchestrator looks up. Include a migration script for existing buckets.

[cursor]

PR Summary

Low Risk
Low risk: changes are limited to build artifact path naming and documentation, with a backwards-compatible x86_64 legacy copy. Main risk is downstream automation expecting the old x86_64/ directory layout.

Overview
Normalizes kernel build output directories to Go/OCI arch names (amd64, arm64) by mapping TARGET_ARCH=x86_64 to amd64 in build.sh, while still writing a legacy builds/vmlinux-<version>/vmlinux.bin copy for x86_64 compatibility.

Adds migrate-gcs-arch.sh to dry-run/copy (and optionally delete) existing GCS objects from x86_64/ to amd64/, and updates README.md to document the new output layout and arch naming conventions.

^{Reviewed by Cursor Bugbot for commit 63b1dd0. Bugbot is set up for automated code reviews on this repo. Configure here.}