PR18465: [Thumbv8] add predefined macros

currently, for thumbv8, two predefined macros are missing:
 define __THUMB_INTERWORK__ 1
 define __THUMB_INTERWORK__ 1

This patch adds them for thumbv8.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199819 91177308-0d34-0410-b5e6-96231b3b80d8