Fix #1563807: _ctypes built on AIX fails with ld ffi error.

The contents of ffi_darwin.c must be compiled unless __APPLE__ is
defined and __ppc__ is not.

Backport from trunk.
diff --git a/Misc/NEWS b/Misc/NEWS
index 2d790f2..2e5cada 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -70,6 +70,8 @@
 Extension Modules
 -----------------
 
+- Bug #1563807: _ctypes built on AIX fails with ld ffi error.
+
 - Bug #1598620: A ctypes Structure cannot contain itself.
  
 - Bug #1588217: don't parse "= " as a soft line break in binascii's