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

[Tom Vijlbrief via llvm-commits]

tomtor wrote:

Regarding gcc output, compile any C fragment with gcc with eg -mmcu=attiny85 and -mmcu=attiny1614 and compare the start and end of the different -S output.

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