Try fixing Apple cross-compiling

Based on https://github.com/google/boringssl/blob/897a2ca3f184b34278641138c726ef902ab1fab2/CMakeLists.txt#L443-L455

CMakeLists.txt

@@ -77,6 +77,19 @@ include(GNUInstallDirs)
 include(AbseilDll)
 include(AbseilHelpers)
 
+# Apple cross-compiling support takes an architecture list from
+# CMAKE_OSX_ARCHITECTURES, leaves CMAKE_SYSTEM_PROCESSOR alone, and
+# expects all architecture-specific logic to be conditioned within the
+# source files rather than the build. This does not work for our
+# hardware AES runtime support detection, so we fix
+# CMAKE_SYSTEM_PROCESSOR and only support single-architecture builds.
+if(CMAKE_OSX_ARCHITECTURES)
+  list(LENGTH CMAKE_OSX_ARCHITECTURES NUM_ARCHES)
+  if(NOT ${NUM_ARCHES} EQUAL 1)
+    message(FATAL_ERROR "Universal binaries not supported")
+  endif()
+  list(GET CMAKE_OSX_ARCHITECTURES 0 CMAKE_SYSTEM_PROCESSOR)
+endif()
 
 ##
 ## Using absl targets