This PR adds a new Build Config `rv-virt:leds64_rust` that builds the Rust App `leds_rust` for QEMU RISC-V 64-bit. The build requires the [Rust Target `riscv64gc-unknown-none-elf`](https://github.com/apache/nuttx/pull/12858):
```bash
rustup target add riscv64gc-unknown-none-elf
```
New Files:
`boards/risc-v/qemu-rv/rv-virt/configs/leds64_rust/defconfig`: Add new Build Config `rv-virt:leds64_rust`
Updated Docs:
`applications/examples/leds_rust/index.rst`: Add `leds_rust` example app
`platforms/risc-v/qemu-rv/boards/rv-virt/index.rst`: Add `rv-virt:leds64_rust` config
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