[AVR] Do not clear r0 at interrupt entry
There is no reason to do this: it's a scratch register and can therefore
hold any arbitrary value. And because it is in an interrupt, this code
is performance critical so it should be as short as possible.
I believe r0 was cleared because of the following:
1. There used to be a bug that the cleared register was r0, not r1 as
it should have been.
2. This was fixed in https://reviews.llvm.org/D99467, but left the code
to clear r0.
This patch completes D99467 by removing the `clr r0` instruction.
Differential Revision: https://reviews.llvm.org/D116756