Attempt to fix Linux build - __has_attribute(swiftcall) doesn't work on Linux

Author: hughbe

include/swift/Runtime/Config.h

@@ -35,7 +35,7 @@
 /// Does the current Swift platform use LLVM's intrinsic "swiftcall"
 /// calling convention for Swift functions?
 #ifndef SWIFT_USE_SWIFTCALL
-#if __has_attribute(swiftcall)
+#if __has_attribute(swiftcall) || defined(__linux__)
 #define SWIFT_USE_SWIFTCALL 1
 #else
 #define SWIFT_USE_SWIFTCALL 0