[AVR] Fix stack size in functions with a frame pointer

This patch fixes a bug in stack save/restore code. Because the frame
pointer was saved/restored manually (not by marking it as clobbered) the
StackSize variable was not updated accordingly. Most code still worked,
but code that tried to load a parameter passed on the stack did not.

This commit fixes this by marking the frame pointer as a
callee-clobbered register. This will let it be saved without any effort
in prolog/epilog code and will make sure the correct address is
calculated for loading parameters that are passed on the stack.

This approach is used by most other targets (such as X86, AArch64 and
RISC-V).

Differential Revision: https://reviews.llvm.org/D78579