[AVR] Mark call-clobbered registers as clobbered in interrupt handlers

I have matched the RISCV backend, which only uses the interrupt save
list in getCalleeSavedRegs, _not_ in getCallPreservedMask. I don't know
the details of these two methods, but with it, the correct amount of
registers is saved and restored.

Without this patch, practically all interrupt handlers that call a
function will miscompile.

I have added a test to verify this behavior. I've also added a very
simple test to verify that more normal interrupt operations (in this
case, incrementing a global value) behave as expected.

Differential Revision: https://reviews.llvm.org/D116551