Don't define SWIFT_USE_SWIFTCALL if there is no such attribute

Author: hughbe

include/swift/Runtime/Config.h

@@ -35,10 +35,10 @@
 /// Does the current Swift platform use LLVM's intrinsic "swiftcall"
 /// calling convention for Swift functions?
 #ifndef SWIFT_USE_SWIFTCALL
-#ifdef __s390x__
+#if __has_attribute(swiftcall)
 #define SWIFT_USE_SWIFTCALL 1
 #else
-#define SWIFT_USE_SWIFTCALL 1
+#define SWIFT_USE_SWIFTCALL 0
 #endif
 #endif