tooling: add tools/compare_v_performance_between_commits

easily compare v performance/size across commits.

* fix eprintln on linux (it now uses stderr, and flushes it).

* flag: cleaner usage information.

vlib/builtin/builtin.v

@@ -91,14 +91,19 @@ pub fn println(s string) {
 pub fn eprintln(s string) {
 	if isnil(s.str) {
 		panic('eprintln(NIL)')
-	}
+	}  
 	$if mac {
 		C.fprintf(stderr, '%.*s\n', s.len, s.str)
-	}
+		C.fflush(stderr)
+		return 
+	} 
+	$if linux {
+		C.fprintf(stderr, '%.*s\n', s.len, s.str)
+		C.fflush(stderr)
+		return
+	}  
 	// TODO issues with stderr and cross compiling for Linux
-	$else {
-		println(s)
-	}
+	println(s)
 }
 
 pub fn print(s string) {