Add avr_target_feature
This adds the following unstable target features (tracking issue: https://github.com/rust-lang/rust/issues/146889):
- The following two are particularly important for properly supporting inline assembly:
- `tinyencoding`: AVR has devices that reduce the number of registers, similar to RISC-V's RV32E. This feature is necessary to support inline assembly in such devices. (see also https://github.com/rust-lang/rust/pull/146901)
- `lowbytefirst`: AVR's memory access is per 8-bit, and when writing 16-bit ports, the bytes must be written in a specific order. This order depends on devices, making this feature necessary to write proper inline assembly for such use cases. (see also 2a528760bf)
- The followings help recognizing whether specific instructions are available:
- `addsubiw`
- `break`
- `eijmpcall`
- `elpm`
- `elpmx`
- `ijmpcall`
- `jmpcall`
- `lpm`
- `lpmx`
- `movw`
- `mul`
- `rmw`
- `spm`
- `spmx`
Of these, all except `addsubiw`, `break`, `ijmpcall`, `lpm`, `rmw`, `spm`, and `spmx` have [corresponding conditional codes in avr-libc](https://github.com/search?q=repo%3Aavrdudes%2Favr-libc+%2F__AVR_HAVE_%2F&type=code&p=1). LLVM also has `des` feature, but I excluded it from this PR because [DES](https://en.wikipedia.org/wiki/Data_Encryption_Standard) is insecure.
- Report future-incompatible warning (https://github.com/rust-lang/rust/issues/116344) for -C target-feature=-sram and -C target-cpu=<device_without_sram> cases because SRAM is minimum requirement for non-assembly language in both avr-gcc and LLVM.
- See https://github.com/rust-lang/rust/pull/146900#issuecomment-3323558005 for details.
LLVM also has `smallstack`, `wrappingrjmp`, and `memmappedregs` features, but I skipped them because they didn't seem to belong to either of the above categories, but I might have missed something.
(The feature names are match with [definitions in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-21.1.0/llvm/lib/Target/AVR/AVRDevices.td).)
cc @Patryk27 @Rahix
r? workingjubilee
@rustbot label +O-AVR +A-target-feature
d7c812cb57