build/cmake: Fix CMP0023 warning with libffi

Fixes:
CMake Warning (dev) at lib/ExecutionEngine/Interpreter/CMakeLists.txt:16 (target_link_libraries):
  Policy CMP0023 is not set: Plain and keyword target_link_libraries
  signatures cannot be mixed.  Run "cmake --help-policy CMP0023" for policy
  details.  Use the cmake_policy command to set the policy and suppress this
  warning.

  The keyword signature for target_link_libraries has already been used with
  the target "LLVMInterpreter".  All uses of target_link_libraries with a
  target should be either all-keyword or all-plain.

  The uses of the keyword signature are here:

   * cmake/modules/AddLLVM.cmake:345 (target_link_libraries)

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 217154