[LLVM][AVR] Support for R_AVR_6 fixup

Summary: Handle the emission of `R_AVR_6` ELF relocation type.

Reviewers: dylanmckay

Reviewed By: dylanmckay

Subscribers: hiraditya, Jim, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78721

Patch by @LemonBoy https://reviews.llvm.org/p/LemonBoy/