Fix compilation of unwind on Darwin-x86_64

EHABI related typedef sugar is gated via LIBCXXABI_ARM_EHABI which did not
protect the EHABI header.  This would cause declarations to be emitted on
non-EHABI targets, resulting in errors.  This permits compilation on Darwin.

llvm-svn: 228359
diff --git a/libcxxabi/src/Unwind/Unwind-EHABI.h b/libcxxabi/src/Unwind/Unwind-EHABI.h
index f55b71f..2011a67 100644
--- a/libcxxabi/src/Unwind/Unwind-EHABI.h
+++ b/libcxxabi/src/Unwind/Unwind-EHABI.h
@@ -14,6 +14,8 @@
 #include <stdint.h>
 #include <unwind.h>
 
+#if LIBCXXABI_ARM_EHABI
+
 // Unable to unwind in the ARM index table (section 5 EHABI).
 #define UNW_EXIDX_CANTUNWIND 0x1
 
@@ -42,4 +44,6 @@
 } // extern "C"
 #endif
 
+#endif
+
 #endif  // __UNWIND_EHABI_H__