message: Update miniapps/performance/CMakeLists.txt to check for ppc64 arch, but w/o mcpu option
diff: # terms of the BSD-3 license. We welcome feedback and contributions, see file
  # CONTRIBUTING.md for details.
  +if (${CMAKE_SYSTEM_PROCESSOR} MATCHES "ppc64")
  + set(MFEM_PERF_CXX_ARCH_FLAGS "-mtune=native") # Unsupported mcpu option
  +else()
  + set(MFEM_PERF_CXX_ARCH_FLAGS "-march=native")
  +endif()
  +
  set(PERFORMANCE_CXX_OPTIONS)
  if (${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
  list(APPEND PERFORMANCE_CXX_OPTIONS
  - "-march=native"
  + ${MFEM_PERF_CXX_ARCH_FLAGS}
  "-Wpedantic"
  "-Wall"
  "-fcolor-diagnostics"
  @@ -22,7 +28,7 @@ if (${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
  "-ffp-contract=fast")
  elseif (${CMAKE_CXX_COMPILER_ID} MATCHES "GNU")
  list(APPEND PERFORMANCE_CXX_OPTIONS
  - "-march=native"
  + ${MFEM_PERF_CXX_ARCH_FLAGS}
  "-Wall"
  "--param" "max-completely-peel-times=3")
  if (NOT MFEM_USE_CUDA)
  