Silence some -Wundef warnings

config.h:53:7: warning 'FOR_DYLD' is not defined, evaluates to 0 [-Wundef]
Unwind_AppleExtras.cpp:44:5: warning '__arm__' is not defined, evaluates to 0 [-Wundef]
Unwind_AppleExtras.cpp:60:7: warning '__arm64__' is not defined, evaluates to 0 [-Wundef]
Unwind_AppleExtras.cpp:186:6: warning 'FOR_DYLD' is not defined, evaluates to 0 [-Wundef]

Use defined(macro) which should be equivalent in these cases, silencing -Wundef
warnings.  NFC.

llvm-svn: 228358