risc-v/qemu-rv: Add Build Config for leds64_rust

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`](apache#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

Documentation/applications/examples/leds_rust/index.rst

@@ -0,0 +1,6 @@
+=================================
+``leds_rust`` Toggle LEDs in Rust
+=================================
+
+This is a simple test in Rust of the board LED driver at
+``nuttx/drivers/leds/userled_*.c``.

Documentation/platforms/risc-v/qemu-rv/boards/rv-virt/index.rst

@@ -257,6 +257,11 @@ leds64
 
 Similar to the `nsh64`_ configuration, but with User LEDs support for 64-bit RISC-V.
 
+leds64_rust
+-----------
+
+Similar to the `leds64`_ configuration, but with ``leds_rust`` example enabled.
+
 netnsh
 ------
 

boards/risc-v/qemu-rv/rv-virt/configs/leds64_rust/defconfig

@@ -0,0 +1,76 @@
+#
+# This file is autogenerated: PLEASE DO NOT EDIT IT.
+#
+# You can use "make menuconfig" to make any modifications to the installed .config file.
+# You can then do "make savedefconfig" to generate a new defconfig file that includes your
+# modifications.
+#
+# CONFIG_DISABLE_OS_API is not set
+# CONFIG_NSH_DISABLE_LOSMART is not set
+CONFIG_16550_ADDRWIDTH=0
+CONFIG_16550_UART0=y
+CONFIG_16550_UART0_BASE=0x10000000
+CONFIG_16550_UART0_CLOCK=3686400
+CONFIG_16550_UART0_IRQ=37
+CONFIG_16550_UART0_SERIAL_CONSOLE=y
+CONFIG_16550_UART=y
+CONFIG_ARCH="risc-v"
+CONFIG_ARCH_BOARD="rv-virt"
+CONFIG_ARCH_BOARD_QEMU_RV_VIRT=y
+CONFIG_ARCH_CHIP="qemu-rv"
+CONFIG_ARCH_CHIP_QEMU_RV64=y
+CONFIG_ARCH_CHIP_QEMU_RV=y
+CONFIG_ARCH_CHIP_QEMU_RV_ISA_A=y
+CONFIG_ARCH_CHIP_QEMU_RV_ISA_C=y
+CONFIG_ARCH_CHIP_QEMU_RV_ISA_M=y
+CONFIG_ARCH_INTERRUPTSTACK=2048
+CONFIG_ARCH_RISCV=y
+CONFIG_ARCH_STACKDUMP=y
+CONFIG_BCH=y
+CONFIG_BOARDCTL_POWEROFF=y
+CONFIG_BOARD_LATE_INITIALIZE=y
+CONFIG_BOARD_LOOPSPERMSEC=6366
+CONFIG_BUILTIN=y
+CONFIG_DEBUG_FEATURES=y
+CONFIG_DEBUG_FULLOPT=y
+CONFIG_DEBUG_SYMBOLS=y
+CONFIG_DEVICE_TREE=y
+CONFIG_DEV_ZERO=y
+CONFIG_ELF=y
+CONFIG_EXAMPLES_HELLO=y
+CONFIG_EXAMPLES_LEDS=y
+CONFIG_EXAMPLES_LEDS_RUST=y
+CONFIG_FS_HOSTFS=y
+CONFIG_FS_PROCFS=y
+CONFIG_IDLETHREAD_STACKSIZE=2048
+CONFIG_INIT_ENTRYPOINT="nsh_main"
+CONFIG_INIT_STACKSIZE=3072
+CONFIG_LIBC_ENVPATH=y
+CONFIG_LIBC_EXECFUNCS=y
+CONFIG_LIBC_PERROR_STDOUT=y
+CONFIG_LIBC_STRERROR=y
+CONFIG_LIBM=y
+CONFIG_NFILE_DESCRIPTORS_PER_BLOCK=6
+CONFIG_NSH_ARCHINIT=y
+CONFIG_NSH_BUILTIN_APPS=y
+CONFIG_NSH_FILEIOSIZE=512
+CONFIG_NSH_READLINE=y
+CONFIG_PATH_INITIAL="/system/bin"
+CONFIG_RAM_SIZE=33554432
+CONFIG_RAM_START=0x80000000
+CONFIG_READLINE_CMD_HISTORY=y
+CONFIG_RISCV_SEMIHOSTING_HOSTFS=y
+CONFIG_RR_INTERVAL=200
+CONFIG_SCHED_WAITPID=y
+CONFIG_SERIAL_UART_ARCH_MMIO=y
+CONFIG_STACK_COLORATION=y
+CONFIG_START_MONTH=12
+CONFIG_START_YEAR=2021
+CONFIG_SYMTAB_ORDEREDBYNAME=y
+CONFIG_SYSTEM_NSH=y
+CONFIG_SYSTEM_NSH_STACKSIZE=3072
+CONFIG_TESTING_GETPRIME=y
+CONFIG_TESTING_OSTEST=y
+CONFIG_USEC_PER_TICK=1000
+CONFIG_USERLED=y
+CONFIG_USERLED_LOWER=y