[llvm] [AVR] No cli for SPWRITE on XMEGA (PR #147210)

[Tom Vijlbrief via llvm-commits]

tomtor wrote:

> For the unit test to be supplemented, a contrast test between `avr6` and `avrxemaga3` is enough.

I added avr128db28 and attiny1614. Note that we MUST test for SPH to prevent writing wrong assembly with -1 as destination, so I added that single test.

So we have 3 tests, a classic (atmega328), a tiny without SPH and some modern xmegas/tiny/avr

https://github.com/llvm/llvm-project/pull/147210