CMake: metal C files not handled properly in omrport

[dnakamura]

The way we have z/OS metal c files implemented uses add_custom_command to generate a .o file from the .mc. However, CMake does not allow adding object files to object_libraries

CMake Error at /home/jenkins/devinn/openj9-openjdk-jdk11-zos/omr/cmake/modules/OmrTargetSupport.cmake:70 (add_library):
  OBJECT library "omrport_obj" contains:

    omrlpdat.o

  but may contain only sources that compile, header files, and other files
  that would not affect linking of a normal library.

Why this doesnt trigger an error when building omr, I have no idea.