[mach-o] Support data-only dylibs

In final linked shared images, the __TEXT segment contains both code and
the mach-o header/load-commands.  In the case of a data-only dylib, there is
no code, so we need to force the addition of the __TEXT segment.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220597 91177308-0d34-0410-b5e6-96231b3b80d8