chore(arm64): make the BPF build arch-aware for aarch64

[tattoosonmyskin]

Prep for running Jinn Guard on arm64. No change to x86 output — ARCH still resolves to x86 on x86_64 (verified: ARCH=[x86] UNAME_M=[x86_64]), and an LSM object still compiles identically. This just stops the build hardcoding x86 so an aarch64 host builds without manual flags.

• bpf/Makefile: derive ARCH from uname -m (x86_64->x86, aarch64->arm64), overridable with make ARCH=..., defaulting to x86 for unknown machines. Adds a regen-vmlinux target — the committed bpf/vmlinux.h is x86-generated (vendored so CI compiles without BTF), so a foreign-arch build regenerates it from local BTF.
• scripts/validate_m2_path_resolution.sh, scripts/run_professor_validation.sh: derive -D__TARGET_ARCH_ from the host instead of hardcoding _x86.
• rust-toolchain.toml: add aarch64-unknown-linux-gnu to the target list.

Does not touch the x86-hosted CI compile-gate (correct as-is). Real on-board bring-up (BTF gate, runner registration, kernel_lsm validation) still needs the aarch64 hardware and is tracked in the internal bring-up runbook.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR makes the eBPF (BPF-LSM) build and validation flow architecture-aware so the project can be built on arm64/aarch64 hosts without manually editing flags, while preserving the existing x86_64 behavior.

Changes:

• Derive the kernel-style BPF arch name (x86 vs arm64) from uname -m in the BPF Makefile (with ARCH still overridable via make ARCH=...).
• Update validation scripts to pass -D__TARGET_ARCH_${arch} instead of hardcoding __TARGET_ARCH_x86.
• Add aarch64-unknown-linux-gnu to the Rust toolchain target list.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
bpf/Makefile	Makes ARCH host-derived (x86_64→x86, aarch64→arm64) and adds a regen-vmlinux helper target for local BTF regeneration.
scripts/validate_m2_path_resolution.sh	Uses host-derived __TARGET_ARCH_* define when compiling BPF LSM objects during validation.
scripts/run_professor_validation.sh	Uses host-derived __TARGET_ARCH_* define when compiling/installing BPF LSM objects in Tier 4.
rust-toolchain.toml	Adds the aarch64 Linux target to the pinned toolchain configuration.

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