[lld-macho] Export trie addresses should be relative to the image base

We didn't notice this earlier this we were only testing the export trie
encoded in a dylib, whose image base starts at zero. But a regular
executable contains `__PAGEZERO`, which means it has a non-zero image
base. This bug was discovered after attempting to run some programs that
performed `dlopen` on an executable.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D87780