Backport "[AVR] Fix parsing & emitting relative jumps"

[aykevl]

/cherry-pick 86a60e7

This fix is critical to get the AVR backend working again. Without this fix, it's practically unusable because almost all branches have an off-by-one error (one word, 2 bytes). For more context, see #102436 and #102936.

CC @Patryk27

[Patryk27]

Should I prepare the backport myself or rather there's some magic llvm bot that'll do it? 😄

[llvmbot]

/pull-request #106729

[aykevl]

Well there you see it, there's a magic LLVM bot for it :)