riscv: fix RV64 CSR compilation

[siju-felsite]

Pull Request Overview

The RV64 cfg-gating in arch/riscv/src/csr/mod.rs was never compiled for a riscv64 target (no RV64 board exists yet), which hid two bugs that stopped the riscv crate from building for riscv64:

1. pmpcfg0 was gated cfg(not(target_arch = "riscv64")), removing it on RV64. RV64 uses the even-indexed PMP config registers (pmpcfg0, pmpcfg2, …, pmpcfg14); pmpcfg0 is the first of these and must be present. The const CSR initializer and all three pmpconfig_{get,set,modify} accessors already reference pmpcfg0 unconditionally, so the missing field caused E0560/E0609.
2. read_cycle_counter() on RV64 returned the usize result of CSR.mcycle.read(...) where u64 is expected (E0308); added an explicit cast.

This also annotates the CSR id import with cfg_attr(target_arch = "riscv64", allow(unused_imports)), since the high-half (*H) and odd-indexed PMPCFG* registers are cfg'd out on riscv64.

The rv32 code paths are untouched. This is groundwork toward #2332 (64-bit RISC-V support) and is independent of any board or the remaining startup/trap/context-switch asm.

Testing Strategy

Built arch/riscv (and a thin downstream rv64 crate that depends on it) for riscv64imac-unknown-none-elf — it now compiles cleanly and warning-free, whereas before it failed with the errors above. make prepush (format-check, clippy, syntax, licensecheck) passes clean. The rv32 cfg paths are unchanged, so existing RISC-V boards are unaffected.

TODO or Help Wanted

None — this is a standalone build-correctness fix.

Documentation Updated

• Updated the relevant files in /docs, or no updates are required.

Formatting

• Ran make prepush.

AI Use

This change was developed with the assistance of an AI coding assistant (Claude / Claude Code). The two compile-error fixes and the cfg_attr(allow(unused_imports)) annotation were AI-generated and then reviewed before submission.

• The PR description details my use of AI in the production of the code in this PR, if any, and I have manually checked and personally certify the entire contents of this PR.