[lld/mac] Partially implement -export_dynamic

This implements the part of -export_dynamic that adds external
symbols as dead strip roots even for executables.

It does not yet implement the effect -export_dynamic has for LTO.
I tried just replacing `config->outputType != MH_EXECUTE` with
`(config->outputType != MH_EXECUTE || config->exportDynamic)` in
LTO.cpp, but then local symbols make it into the symbol table too,
which is too much (and also doesn't match ld64). So punt on this
for now until I understand it better.
(D91583 may or may not be related too).

Differential Revision: https://reviews.llvm.org/D105482