[lld-macho] Increase slop to prevent thunk out of range again.

Building Firefox with -O0 on arm64 mac recently hit the
"FIXME: thunk range overrun" error on multiple occasions.

Doubling or tripling slop was not sufficient in some cases, so
quadruple it.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D138174