Switch to built-in targets

[matteocarnelos]

Hi!

I'm wondering why this HAL still relies on custom targets for the AVR devices instead of using the compiler's built-in ones.
The problem with custom targets is that a lot of tools don't support them.
In particular, the following are some tools that I'm using that don't support them:

• IntelliJ Rust emits a warning when syncing the project
• cargo-binutils' objdump command throws the error: Could not find specification for target "avr-atmega328p"
• cargo-call-stack throws the error: error: couldn't open ELF file 'avr-hal/target/../../avr-specs/avr-atmega328p.json/release/uno-blink': No such file or directory (os error 2)
• According to this comment:

  [...] compiler-builtins (and several other crates) do sniffing based on the target name, which obviously won't work with JSON spec targets. [...]

As of now, it's possible to use the built-in avr-unknown-gnu-atmega328 target, together with the default-linker-libraries flag, to successfully run avr-hal applications on ATmega328-based boards (e.g. Arduino Uno), without all the errors listed above.

So the question is: why don't we add all the targets in the avr-specs directory to the compiler and use them?
This would not only solve all the issues above, but it would also make the development of avr-hal applications more seamless.

By doing a quick search into the Rust repo, I was only able to find this comment, saying that, as of July 2020:

[...] There is no current intention to add in-tree AVR target specifications for other
AVR microcontrollers [...]

But I couldn't find any reason for this decision.