[sanitizers test CMake] further refactor testing CMake for tsan

TSan tests on Darwin first link all libraries into a static archive file.
With this change, the linking is done once per all architecture,
and previously the linking step was repeated per each architecture per
each add_tsan_test call.
Furthermore, the code is cleared up.

Differential Revision: https://reviews.llvm.org/D35913

llvm-svn: 309406