r316368 broke this build when it introduced a reference to a pthread
function to the Utility module. This caused cmake to generate an
incorrect link line (wrong order of libs) because it did not see the
dependency from Utility to the system libraries. Instead these libraries
were being manually added to each final target.
This changes moves the dependency management from the individual targets
to the lldbUtility module, which is consistent with how llvm does it.
The final targets will pick up these libraries as they will be a part of
the link interface of the module.
Technically, some of these dependencies could go into the host module,
as that's where most of the os-specific code is, but I did not try to
investigate which ones.