vlib: use the builtin flush functions, instead of the C. ones (#20108)

vlib/builtin/builtin.c.v

@@ -171,14 +171,14 @@ pub fn eprintln(s string) {
 	} $else $if ios {
 		C.WrappedNSLog(s.str)
 	} $else {
-		C.fflush(C.stdout)
-		C.fflush(C.stderr)
+		flush_stdout()
+		flush_stderr()
 		// eprintln is used in panics, so it should not fail at all
 		$if android && !termux {
 			C.android_print(C.stderr, c'%.*s\n', s.len, s.str)
 		}
 		_writeln_to_fd(2, s)
-		C.fflush(C.stderr)
+		flush_stderr()
 	}
 }
 
@@ -195,13 +195,13 @@ pub fn eprint(s string) {
 		// TODO: Implement a buffer as NSLog doesn't have a "print"
 		C.WrappedNSLog(s.str)
 	} $else {
-		C.fflush(C.stdout)
-		C.fflush(C.stderr)
+		flush_stdout()
+		flush_stderr()
 		$if android && !termux {
 			C.android_print(C.stderr, c'%.*s', s.len, s.str)
 		}
 		_write_buf_to_fd(2, s.str, s.len)
-		C.fflush(C.stderr)
+		flush_stderr()
 	}
 }
 

vlib/os/os.c.v

@@ -399,9 +399,9 @@ pub fn system(cmd string) int {
 	$if windows {
 		// overcome bug in system & _wsystem (cmd) when first char is quote `"`
 		wcmd := if cmd.len > 1 && cmd[0] == `"` && cmd[1] != `"` { '"${cmd}"' } else { cmd }
+		flush_stdout()
+		flush_stderr()
 		unsafe {
-			C.fflush(C.stdout)
-			C.fflush(C.stderr)
 			ret = C._wsystem(wcmd.to_wide())
 		}
 	} $else {

vlib/readline/readline_nix.c.v

@@ -121,7 +121,7 @@ pub fn (mut r Readline) read_line_utf8(prompt string) ![]rune {
 	}
 	print(r.prompt)
 	for {
-		unsafe { C.fflush(C.stdout) }
+		flush_stdout()
 		c := r.read_char() or { return err }
 		a := r.analyse(c)
 		if r.execute(a, c) {

vlib/readline/readline_windows.c.v

@@ -36,7 +36,7 @@ pub fn (mut r Readline) read_line_utf8(prompt string) ![]rune {
 		r.previous_lines[0] = []rune{}
 	}
 	print(r.prompt)
-	unsafe { C.fflush(C.stdout) }
+	flush_stdout()
 	r.current = os.get_raw_line().runes()
 	r.previous_lines[0] = []rune{}
 	r.search_index = 0