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

[stefansf at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121343
Bug 121343 depends on bug 121198, which changed state.

Bug 121198 Summary: [avr][hreg-constraints] error: 'asm' operand has impossible constraints or there are not enough registers
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121198

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