feat: add percpu

[AsakuraMizu]

Move part of axhal::percpu from ArceOS into axplat.

Useful for PLIC.

axcpu::init::init_percpu(x86_64) will be removed later.

[Copilot]

Pull request overview

This PR introduces a new axplat::percpu module to provide common CPU-local (per-CPU) helpers inside axplat, and updates example kernels to use the new initialization and access APIs (removing the x86_64-specific axcpu::init::init_percpu call in examples).

Changes:

• Add axplat::percpu with per-CPU CPU_ID/IS_BSP storage plus init_primary/init_secondary.
• Update hello-kernel, irq-kernel, and smp-kernel examples to initialize per-CPU state via axplat::percpu and read CPU ID via axplat::percpu::this_cpu_id().
• Adjust hello-kernel linker script to include linkme_* sections and drop the direct axcpu dependency for x86_64.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
platforms/axplat-riscv64-qemu-virt/src/irq.rs	Minor import formatting change.
examples/smp-kernel/src/irq.rs	Switch timer IRQ log to use axplat::percpu::this_cpu_id().
examples/smp-kernel/src/init.rs	Replace local per-CPU CPU_ID plumbing with axplat::percpu::init_primary/init_secondary.
examples/irq-kernel/src/main.rs	Use axplat::percpu::init_primary instead of x86_64-only axcpu percpu init.
examples/hello-kernel/src/main.rs	Use axplat::percpu::init_primary unconditionally.
examples/hello-kernel/linker.lds.S	Add linkme_* sections (IRQ + PAGE_FAULT) inserted after .data.
examples/hello-kernel/Cargo.toml	Remove x86_64-only axcpu dependency.
axplat/src/percpu.rs	New per-CPU API surface (CPU ID + BSP flag + init helpers).
axplat/src/lib.rs	Export the new percpu module.
axplat/Cargo.toml	Add percpu crate dependency.
Cargo.lock	Lockfile updates for new dependency graph.

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[equation314]

These sections are unnecessary for hello-kernel.

[AsakuraMizu]

Removing it will cause builds to fail on aarch64 and riscv64, just like in https://github.com/arceos-org/axplat_crates/actions/runs/21898792110?pr=42. I don't actually know why.

[equation314]

Add a feature for percpu? Introducing percpu requires modifying the linker script, which some kernels may not want.