Add avr-none triple support (bazelbuild#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.

Author: stefanbucur

rust/platform/triple.bzl

@@ -40,6 +40,14 @@ def triple(triple):
             abi = None,
             str = triple,
         )
+    elif triple == "avr-none":
+        return struct(
+            arch = "avr",
+            vendor = None,
+            system = "none",
+            abi = None,
+            str = triple,
+        )
     elif triple in ("aarch64-fuchsia", "x86_64-fuchsia"):
         return struct(
             arch = triple.split("-")[0],

test/unit/platform_triple/platform_triple_test.bzl

@@ -144,6 +144,9 @@ def _construct_known_triples_test_impl(ctx):
     # WebAssembly MVP target
     _assert_parts(env, triple("wasm32v1-none"), "wasm32", "v1", "none", None)
 
+    # AVR bare-metal target
+    _assert_parts(env, triple("avr-none"), "avr", None, "none", None)
+
     _assert_parts(env, triple("x86_64-fuchsia"), "x86_64", "unknown", "fuchsia", None)
     _assert_parts(env, triple("x86_64-apple-ios-macabi"), "x86_64", "apple", "ios", "macabi")