[Clang][Darwin] Define __ARM_DWARF_EH__ for WatchABI

Summary: The Darwin armv7k ABI uses Dwarf EH, so we need to set the OS define correctly. Without this the gcc_personality fails to build.

Reviewers: t.p.northover

Subscribers: aemerson, cfe-commits, rengolin

Differential Revision: http://reviews.llvm.org/D19693

llvm-svn: 268078