Fix CompilationUnit constructor issues.
Fix CompilationUnit constructor issues.

Ensure target64 is initialized. Switch from NULL to nullptr as the former gives
compilation errors with std::unique_ptr from libc++.

Change-Id: I7153368d9324d10ef257f7c7ce9571a1753e5ba8

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst