tools/rust: Fix aarch64 NuttX Rust target specs

[toku-mac]

Summary

Previously, sim builds on Apple Silicon used aarch64-apple-darwin as
the Rust target, which sets target_os=macos and breaks NuttX-specific
conditional compilation.

This PR introduces tools/aarch64-unknown-nuttx-macho.json, a custom
Rust target that sets os=nuttx while producing Mach-O objects
compatible with the macOS simulator linker. tools/Rust.defs is updated
to select this target when building sim on macOS.

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 arch/sim: Translate Apple clock IDs for macOS sim #18978

Configuration and Build:

rm -rf build-debug
make distclean
cmake -S . -B build-debug -DBOARD_CONFIG=sim:nsh -GNinja

printf "CONFIG_SYSTEM_TIME64=y
CONFIG_FS_LARGEFILE=y
CONFIG_TLS_NELEM=16
CONFIG_DEV_URANDOM=y
CONFIG_EXAMPLES_HELLO_RUST_CARGO=y
CONFIG_EXAMPLES_HELLO_RUST_CARGO_STACKSIZE=8192
" >> build-debug/.config

cmake --build build-debug -t olddefconfig
cmake --build build-debug

Testing logs before change:

NuttShell (NSH) NuttX-12.13.0
nsh> hello_rust_cargo
{"name":"John","age":30}
{"name":"Jane","age":25}
Deserialized: Alice is 28 years old
Pretty JSON:
{
  "name": "Alice",
  "age": 28
}

thread '<unnamed>' (1797510) panicked at /Users/toku/.rustup/toolchains/nightly-2026-04-29-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/sys/pal/unix/time.rs:107:68:
called `Result::unwrap()` on an `Err` value: Os { code: 0, kind: Uncategorized, message: "Unknown error 0" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Testing logs after change:

NuttShell (NSH) NuttX-12.13.0
nsh> hello_rust_cargo
{"name":"John","age":30}
{"name":"Jane","age":25}
Deserialized: Alice is 28 years old
Pretty JSON:
{
  "name": "Alice",
  "age": 28
}
Hello world from tokio!

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.

[lupyuen]

@no1wudi Could you review this PR? And also this one below thanks :-)

• apps/cmake: Fix aarch64 NuttX Rust target specs nuttx-apps#3510