[lld-macho] Support __dso_handle for C++

The C++ ABI requires dylibs to pass a pointer to __cxa_atexit which does
e.g. cleanup of static global variables. The C++ spec says that the pointer
can point to any address in one of the dylib's segments, but in practice
ld64 seems to set it to point to the header, so that's what's implemented
here.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D83603