[Bug target/121343] [avr] Use hard-register constraints instead of explicit hard-regs in avr.md

[gjl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121343
Bug 121343 depends on bug 121359, which changed state.

Bug 121359 Summary: [avr] Remove last remains of reload
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121359

           What    |Removed                     |Added
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             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED