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: https://llvm.org/svn/llvm-project/libcxxabi/trunk@228806 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed