[clang] [llvm] [AVR] Add many new AVR MCU model definitions (PR #144229)

[Tom Vijlbrief via llvm-commits]

tomtor wrote:

For future reference, the avr16/avr32 devices do have SPM and SPMX features, but the current xmega3 definition has not.

Xmega3 is also used for modern attiny series which do not have SPM(X), so that is correct.

The avr64 devices use xmega2 which has SPM(X) defined, so that is also ok.

I prefer to leave the avr16/avr32 devices unchanged (using xmega3 to be in sync with gcc definitions), because the SPM instructions write flash and users can use asm if they want that feature. 

For me this PR is complete now.

https://github.com/llvm/llvm-project/pull/144229