[test] Verify that x86_64 targets have the cx16 feature on every platform

Author: lorentey

test/IRGen/cx16.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-frontend -target x86_64-unknown-linux-gnu -parse-stdlib -disable-objc-interop %s -module-name main -emit-ir -o - | %FileCheck %s
+// RUN: %target-swift-frontend -target x86_64-unknown-windows-msvc -parse-stdlib -disable-objc-interop %s -module-name main -emit-ir -o - | %FileCheck %s
+// RUN: %target-swift-frontend -target x86_64-unknown-freebsd -parse-stdlib -disable-objc-interop %s -module-name main -emit-ir -o - | %FileCheck %s
+// RUN: %target-swift-frontend -target x86_64-apple-macosx10.9 -parse-stdlib -module-name main %s -emit-ir -o - | %FileCheck %s
+
+// REQUIRES: CODEGENERATOR=X86
+
+public func test() {
+}
+
+// We expect double-wide atomic intrinsics to always be available on x86_64.
+// CHECK: "target-features"="{{.*}}+cx16,