[AVR][Clang] Implement __AVR_HAVE_*__ macros

These macros are defined in avr-gcc and are useful when working with
assembly. For example, startup code needs to copy the contents of .data
from flash to RAM, but should use elpm (instead of lpm) on devices with
more than 64kB flash. Without __AVR_HAVE_ELPM__, there is no way to know
whether the elpm instruction is supported.

This partially fixes https://github.com/llvm/llvm-project/issues/56157.

Differential Revision: https://reviews.llvm.org/D137572