message: Split miniapps/performance/CMakeLists.txt mcpu and mtune options
diff: # CONTRIBUTING.md for details.
  if (${CMAKE_SYSTEM_PROCESSOR} MATCHES "ppc64")
  - set(MFEM_PERF_CXX_ARCH_FLAGS "-mtune=native") # Unsupported mcpu option
  + set(MFEM_PERF_CXX_ARCH_FLAGS "-mcpu=native" "-mtune=native")
  else()
  set(MFEM_PERF_CXX_ARCH_FLAGS "-march=native")
  endif()
  