avr-hal-generic: prefer compilation error for unsupported targets

[tones111]

The build.rs logic is only used to detect a relatively old compiler version (Feb 2022). Since a nightly build is required to target AVR it's unlikely anyone is benefiting from this check. By removing it we reduce dependencies and implementation complexity.

[tones111]

As requested this commit has been split from #619, however, since it depends on the previous change I've stacked it. As discussed here GitHub doesn't really support splitting dependent changes across multiple reviews.

[Rahix]

So far we've tried to keep avr-hal building on non-avr targets. The motivation was mainly to allow running tests on other platforms without having to remove all avr-hal dependencies from an application. Adding a compile-time error would of course break this.

---

On a different note, even if we were to introduce a compile-time error, I do not think that this is the proper way to do it: The resulting error message is extremely confusing and doesn't hint at the underlying problem at all. The proper way would be a crate-global cfg'ed compile_error!(), similar to this one:

avr-hal/mcu/atmega-hal/src/lib.rs

Lines 27 to 48 in 3c08979

	#[cfg(all(
	not(feature = "device-selected"),
	not(feature = "disable-device-selection-error")
	))]
	compile_error!(
	"This crate requires you to specify your target chip as a feature.
	Please select one of the following
	* atmega48p
	* atmega164pa
	* atmega168
	* atmega328p
	* atmega328pb
	* atmega32u4
	* atmega128a
	* atmega1280
	* atmega2560
	* atmega1284p
	* atmega8
	"
	);

The tricky part is ensuring that such an error also short-circuits all other compiler errors that are caused by the same reason, to ensure clean and easy-to-navigate diagnostics.