[AVR] Fix inaccurate offsets in PC relative branch instructions

In avr-gcc, the destination of "rjmp label + offset" is address
'label + offset', while destination of "rjmp . + offset" is
'address_of_rjmp + offset + 2'.

Clang is in accordance with avr-gcc for "rjmp label + offset", but
emits incorrect destination of "rjmp . + offset" to
'address_of_rjmp + offset', in which the expected offset 2 is missing.

This patch fixes the above issue.

Fixes https://github.com/llvm/llvm-project/issues/60019

Reviewed By: jacquesguan, aykevl

Differential Revision: https://reviews.llvm.org/D143901