os: workaround a -prod -cc gcc bug, affecting os.open_file (fix #20923) (related to #20872) (#20960)

vlib/builtin/string.v

@@ -315,7 +315,7 @@ fn (a string) clone_static() string {
 
 // clone returns a copy of the V string `a`.
 pub fn (a string) clone() string {
-	if a.len == 0 {
+	if a.len <= 0 {
 		return ''
 	}
 	mut b := string{

vlib/os/file.c.v

@@ -47,6 +47,7 @@ fn fix_windows_path(path string) string {
 }
 
 // open_file tries to open or create a file with custom flags and permissions.
+@[noinline]
 pub fn open_file(path string, mode string, options ...int) !File {
 	mut flags := 0
 	mut seek_to_end := false

vlib/v/gen/c/testdata/self_printer_with_prod.out

@@ -0,0 +1,15 @@
+// vtest vflags: -prod -skip-unused
+// See https://github.com/vlang/v/issues/20923
+module main
+
+import os
+import io
+
+fn main() {
+	mut file := os.open_file(@FILE, 'r')!
+	mut buff_reader := io.new_buffered_reader(reader: file)
+	for {
+		a := buff_reader.read_line() or { break }
+		println(a)
+	}
+}

vlib/v/gen/c/testdata/self_printer_with_prod.vv

@@ -0,0 +1,15 @@
+// vtest vflags: -prod -skip-unused
+// See https://github.com/vlang/v/issues/20923
+module main
+
+import os
+import io
+
+fn main() {
+	mut file := os.open_file(@FILE, 'r')!
+	mut buff_reader := io.new_buffered_reader(reader: file)
+	for {
+		a := buff_reader.read_line() or { break }
+		println(a)
+	}
+}