[AVR] Rename the last few remaining references from 'avr-unknown-unknown' to 'avr-unknown-gnu-atmega328'

Author: dylanmckay

panic_unwind/src/lib.rs

@@ -65,7 +65,7 @@ cfg_if::cfg_if! {
         // - os=none ("bare metal" targets)
         // - os=uefi
         // - nvptx64-nvidia-cuda
-        // - avr-unknown-unknown
+        // - arch=avr
         #[path = "dummy.rs"]
         mod real_imp;
     }

std/build.rs

@@ -83,7 +83,7 @@ fn main() {
         // - os=none ("bare metal" targets)
         // - mipsel-sony-psp
         // - nvptx64-nvidia-cuda
-        // - avr-unknown-unknown
+        // - arch=avr
         // - tvos (aarch64-apple-tvos, x86_64-apple-tvos)
         // - uefi (x86_64-unknown-uefi, i686-unknown-uefi)
         // - JSON targets