Auto merge of rust-lang#74941 - dylanmckay:replace-broken-avr-unknown-unknown-target, r=oli-obk

[AVR] Replace broken 'avr-unknown-unknown' target with 'avr-unknown-gnu-atmega328' target

The `avr-unknown-unknown` target has never worked correctly, always trying to invoke
the host linker and failing. It aimed to be a mirror of AVR-GCC's
default handling of the `avr-unknown-unknown' triple (assume bare
minimum chip features, silently skip linking runtime libraries, etc).
This behaviour is broken-by-default as it will cause a miscompiled executable
when flashed.

This patch improves the AVR builtin target specifications to instead
expose only a 'avr-unknown-gnu-atmega328' target. This target system is
`gnu`, as it uses the AVR-GCC frontend along with avr-binutils. The
target triple ABI is 'atmega328'.

In the future, it should be possible to replace the dependency on
AVR-GCC and binutils by using the in-progress AVR LLD and compiler-rt support.
Perhaps at that point it would make sense to add an
'avr-unknown-unknown-atmega328' target as a better default when
implemented.

There is no current intention to add in-tree AVR target specifications for other
AVR microcontrollers - this one can serve as a reference implementation
for other devices via `rustc --print target-spec-json
avr-unknown-gnu-atmega328p`.

There should be no users of the existing 'avr-unknown-unknown' Rust
target as a custom target specification JSON has always been
recommended, and the avr-unknown-unknown target could never pass the
linking step anyway.

Author: bors

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