unwind: clean up straggling -Wundef warning

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

llvm-svn: 228806

libcxxabi/src/Unwind/assembly.h

@@ -62,6 +62,10 @@
   SYMBOL_NAME(name):
 
 #if defined(__arm__)
+#if !defined(__ARM_ARCH)
+#define __ARM_ARCH 4
+#endif
+
 #if defined(__ARM_ARCH_4T__) || __ARM_ARCH >= 5
 #define ARM_HAS_BX
 #endif