[Bug target/20296] Speeding up small interrupts on avr

[gjl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20296
Bug 20296 depends on bug 81268, which changed state.

Bug 81268 Summary: [avr] Support __gcc_isr pseudo-instruction for more efficient ISR prologues
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81268

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