message: Change performance ex1 compiler flags for Apple M1
diff: if (${CMAKE_SYSTEM_PROCESSOR} MATCHES "ppc64")
  set(MFEM_PERF_CXX_ARCH_FLAGS "-mcpu=native" "-mtune=native")
  +elseif (APPLE AND ${CMAKE_SYSTEM_PROCESSOR} MATCHES "arm64")
  + set(MFEM_PERF_CXX_ARCH_FLAGS "-mcpu=apple-m1")
  else()
  set(MFEM_PERF_CXX_ARCH_FLAGS "-march=native")
  endif()
  @@ -18,14 +20,13 @@ endif()
  set(PERFORMANCE_CXX_OPTIONS)
  if (${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
  list(APPEND PERFORMANCE_CXX_OPTIONS
  - "-march=native"
  + ${MFEM_PERF_CXX_ARCH_FLAGS}
  "-fcolor-diagnostics"
  "-fvectorize"
  "-fslp-vectorize"
  "-ffp-contract=fast")
  elseif (${CMAKE_CXX_COMPILER_ID} MATCHES "GNU")
  list(APPEND PERFORMANCE_CXX_OPTIONS
  - ${MFEM_PERF_CXX_ARCH_FLAGS}
  "-Wall")
  if (NOT MFEM_USE_CUDA)
  list(APPEND PERFORMANCE_CXX_OPTIONS "-pedantic")
  