Fix wxOSX build with Xcode 10 and later

Update target architectures to support building with the whole range of
supported Xcode versions by not targeting i386 by default, as well as
adding an arm64 target. Targeting i386 results in a deprecation
error starting with Xcode 10:

error: The i386 architecture is deprecated. You should update your ARCHS
build setting to remove the i386 architecture.

docs/changes.txt

@@ -182,6 +182,10 @@ Changes in behaviour which may result in build errors
 - Under macOS, 10.11 SDK is the minimum SDK, building and deploying under 10.10.5 and
   higher is supported, you must use at least Xcode 7.2.1.
 
+- wxOSX Xcode projects no longer include the i386 target by default and,
+  with Xcode 12 or later, build for the arm64 architecture in addition to
+  existing x86_64. See build/osx/wxcocoa.xcconfig for more information.
+
 - wxPGProperty ctors are not longer public since this class is intended to be
   a base class and should not be instantiated directly.