Adding skeleton for unit testing Hexagon Code Emission

Adding and modifying CMakeLists.txt files to run unit tests under
unittests/Target/* if the directory exists.  Adding basic unit test to check
that code emitter object can be retrieved.

Differential Revision: http://reviews.llvm.org/D5523
Change by: Colin LeMahieu

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218986 91177308-0d34-0410-b5e6-96231b3b80d8