Add avr-none triple support (#3950)

**Problem**: The `triple()` parser in `rust/platform/triple.bzl` fails
on `avr-none` because it only has two components separated by `-`, which
does not match the expected `arch-vendor-system[-abi]` format. The
function hits the `fail()` guard at the bottom that requires at least 3
components.

**Solution**: Add a special case for `avr-none` alongside the existing
ones for `wasm32v1-none` and the ARM Thumb targets.

**Additional context**: `avr-none` is the standard Rust target triple
for AVR bare-metal targets (ATtiny, ATmega, AVR-DA series, etc.).
Without this fix, any project using rules_rust with an AVR toolchain
(even a custom one) that sets the target triple to `avr-none` will fail
during toolchain resolution.