[AVR] Add AVR platform support

src/test/ui/feature-gates/feature-gate-abi-avr-interrupt.rs

@@ -0,0 +1,9 @@
+// Test that the AVR interrupt ABI cannot be used when avr_interrupt
+// feature gate is not used.
+
+extern "avr-interrupt" fn foo() {}
+//~^ ERROR avr-interrupt and avr-non-blocking-interrupt ABIs are experimental and subject to change
+
+fn main() {
+    foo();
+}

src/test/ui/feature-gates/feature-gate-abi-avr-interrupt.stderr

@@ -0,0 +1,11 @@
+error[E0658]: avr-interrupt and avr-non-blocking-interrupt ABIs are experimental and subject to change
+  --> $DIR/feature-gate-abi-avr-interrupt.rs:14:1
+   |
+LL | extern "avr-interrupt" fn foo() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(abi_avr_interrupt)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.