macos: support Apple Universal 2 builds (abseil#1086)

fixes abseil#970

absl/copts/AbseilConfigureCopts.cmake

@@ -12,7 +12,49 @@ else()
   set(ABSL_BUILD_DLL FALSE)
 endif()
 
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|amd64|AMD64")
+if(APPLE AND CMAKE_CXX_COMPILER_ID MATCHES [[Clang]])
+  # Some CMake targets (not known at the moment of processing) could be set to
+  # compile for multiple architectures as specified by the OSX_ARCHITECTURES
+  # property, which is target-specific.  We should neither inspect nor rely on
+  # any CMake property or variable to detect an architecture, in particular:
+  #
+  #   - CMAKE_OSX_ARCHITECTURES
+  #     is just an initial value for OSX_ARCHITECTURES; set too early.
+  #
+  #   - OSX_ARCHITECTURES
+  #     is a per-target property; targets could be defined later, and their
+  #     properties could be modified any time later.
+  #
+  #   - CMAKE_SYSTEM_PROCESSOR
+  #     does not reflect multiple architectures at all.
+  #
+  # When compiling for multiple architectures, a build system can invoke a
+  # compiler either
+  #
+  #   - once: a single command line for multiple architectures (Ninja build)
+  #   - twice: two command lines per each architecture (Xcode build system)
+  #
+  # If case of Xcode, it would be possible to set an Xcode-specific attributes
+  # like XCODE_ATTRIBUTE_OTHER_CPLUSPLUSFLAGS[arch=arm64] or similar.
+  #
+  # In both cases, the viable strategy is to pass all arguments at once, allowing
+  # the compiler to dispatch arch-specific arguments to a designated backend.
+  set(ABSL_RANDOM_RANDEN_COPTS "")
+  foreach(_arch IN ITEMS "x86_64" "arm64")
+    string(TOUPPER "${_arch}" _arch_uppercase)
+    string(REPLACE "X86_64" "X64" _arch_uppercase ${_arch_uppercase})
+    foreach(_flag IN LISTS ABSL_RANDOM_HWAES_${_arch_uppercase}_FLAGS)
+      list(APPEND ABSL_RANDOM_RANDEN_COPTS "-Xarch_${_arch}" "${_flag}")
+    endforeach()
+  endforeach()
+  # If a compiler happens to deal with an argument for a currently unused
+  # architecture, it will warn about an unused command line argument.
+  option(ABSL_RANDOM_RANDEN_COPTS_WARNING OFF
+         "Warn if one of ABSL_RANDOM_RANDEN_COPTS is unused")
+  if(ABSL_RANDOM_RANDEN_COPTS AND NOT ABSL_RANDOM_RANDEN_COPTS_WARNING)
+    list(APPEND ABSL_RANDOM_RANDEN_COPTS "-Wno-unused-command-line-argument")
+  endif()
+elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|amd64|AMD64")
   if (MSVC)
     set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_MSVC_X64_FLAGS}")
   else()