[lld] [lld-macho] Fix branch extension thunk estimation logic (PR #120529)

[via llvm-commits]

alx32 wrote:

@nico - could you have a look at this ?
Your previous comment [here](https://reviews.llvm.org/D108924#2972592) is relevant:

> Yes, this computes the end address of all sections. After the loop, isecEnd points right after where the last section would end up, assuming we don't insert thunk sections anywhere along the way.

This fix is addressing the `assuming we don't insert thunk sections anywhere along the way` part - we do end up inserting thunks so this fix ensures  we count them. 

https://github.com/llvm/llvm-project/pull/120529