parser: parse _x64.v files only with -x64

vlib/builtin/float.v

@@ -1,6 +1,5 @@
 // Copyright (c) 2019-2020 Alexander Medvednikov. All rights reserved.
-// Use of this source code is governed by an MIT license
+// Use of this source code is governed by an MIT license that can be found in the LICENSE file.
-// that can be found in the LICENSE file.
 module builtin
 
 import strconv.ftoa

vlib/builtin/float_x64.v

@@ -0,0 +1,7 @@
+// Copyright (c) 2019-2020 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license that can be found in the LICENSE file.
+module builtin
+
+fn float_test()  {
+}
+

vlib/v/pref/should_compile.v

@@ -72,6 +72,9 @@ pub fn (prefs &Preferences) should_compile_c(file string) bool {
 	if file.ends_with('_solaris.c.v') && prefs.os != .solaris {
 		return false
 	}
+	if file.ends_with('_x64.v') && prefs.backend != .x64 {
+		return false
+	}
 	return true
 }