[lld-macho] Stop crash when emitting personalities with -dead_strip

The <internal> symbol was tripping an assertion in getVA() because it
was not marked as used. Per the comment above that symbols creation,
dead stripping has already occurred so marking this symbol as used is
accurate.

Fixes https://github.com/llvm/llvm-project/issues/55565

Differential revision: https://reviews.llvm.org/D126072