[lld-macho] Account for alignment in thunk insertion algorithm

We previously neglected this, leading us to underestimate the maximum
possible branch address offset.

Fixing this should allow us to reduce `slop` to more reasonable
levels. I've lowered it to 256 for now, though I suspect we could go
lower.

Fixes https://github.com/llvm/llvm-project/issues/59259.

Reviewed By: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D144029