unwind: clean up straggling -Wundef warning
unwind: clean up straggling -Wundef warning

Conservatively define __ARM_ARCH to 4 in the case that it is undefined (e.g.
with GCC).

git-svn-id: 6a9f6578bdee8d959f0ed58970538b4ab6004734@228806 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi