runtime: lock output in print/println

This ensures that calls to print/println happening in different threads
are not interleaved. It's a task.PMutex, so this should only change
things when threading is used.

This matches the Go compiler, which does the same thing:
https://godbolt.org/z/na5KzE7en

The locks are not recursive, which means that we need to be careful to
not call `print` or `println` inside a runtime.print* implementation,
inside putchar (recursively), and inside signal handlers. Making them
recursive might be useful to do in the future, but it's not really
necessary.

testdata/print.go

@@ -37,6 +37,12 @@ func main() {
 
 	// print interface
 	println(interface{}(nil))
+	println(interface{}(true))
+	println(interface{}("foobar"))
+	println(interface{}(int64(-3)))
+	println(interface{}(uint64(3)))
+	println(interface{}(int(-3)))
+	println(interface{}(uint(3)))
 
 	// print map
 	println(map[string]int{"three": 3, "five": 5})