[lld/mac] Honor REFERENCED_DYAMICALLY, set it on __mh_execute_header

Has the effect that `__mh_execute_header` stays in the symbol table of
outputs even after running `strip` on the output. I don't know if that's
important for anything -- my motivation for the patch is just is to make
the output more similar to ld64.

(Corresponds to symbolTableInAndNeverStrip in ld64.)

Differential Revision: https://reviews.llvm.org/D102619