apps/cmake: Fix aarch64 NuttX Rust target specs

[toku-mac]

Summary

Previously, aarch64 sim builds on macOS used aarch64-apple-darwin,
which sets target_os=macos and causes NuttX-specific cfg guards to be silently skipped.

This PR adds two custom Rust target JSON files:

• tools/aarch64-unknown-nuttx.json — ELF target for Linux/bare-metal
• tools/aarch64-unknown-nuttx-macho.json — Mach-O target for macOS sim

Rust.mk and cmake/nuttx_add_rust.cmake are updated to select the
appropriate target.

Depends on the corresponding nuttx-side PR. #18992

Impact

Allows proper building of sim on Apple Silicon Macs and aarch64 Linux.
(Support for Intel Macs will be considered separately.)

Testing

I confirm that changes are verified on local setup and works as intended:

• Build Host(s): OS (macOS 26.5), CPU(Apple M1), compiler(Apple clang version 21.0.0)
• Target(s): arch(sim)
• Ensure your PATH environment variable is properly configured to allow execution of: menuconfig, olddefconfig, savedefconfig, and setconfig.
• Use the Rust toolchain version prior to nightly-2026-04-29 to avoid errors related to lib/rustlib/src/rust/library/std/src/sys/net/connection/socket/unix.rs.
• This PR is a successor of PR #18978

PR verification Self-Check

• My PR adheres to Contributing Guidelines and Documentation (git commit title and message, coding standard, etc).
• My PR is ready for review and can be safely merged into a codebase.

[toku-mac]

Since the PR on the nuttx git repository has been merged into master, I've decided not to move the tools/aarch64-unknown-nuttx.json file from the apps git.
Please review commit 6e58319.

[xiaoxiang781216]

Since the PR on the nuttx git repository has been merged into master, I've decided not to move the tools/aarch64-unknown-nuttx.json file from the apps git. Please review commit 6e58319.

but it's better to create a new patch to put all json config in one place.