[AVR] Enable emission of debug information

Prior to this, the code was missing AVR-specific relocation logic in
RelocVisitor.h.

This patch teaches RelocVisitor about R_AVR_16 and R_AVR_32.

Debug information is emitted in the final object file, and understood by
'avr-readelf --debug-dump' from AVR-GCC.

llvm-dwarfdump is yet to understand how to dump AVR DWARF symbols.

llvm-svn: 351720