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.

compiler/compiler.go

@@ -1686,6 +1686,7 @@ func (b *builder) createBuiltin(argTypes []types.Type, argValues []llvm.Value, c
 		b.createRuntimeInvoke("_panic", argValues, "")
 		return llvm.Value{}, nil
 	case "print", "println":
+		b.createRuntimeCall("printlock", nil, "")
 		for i, value := range argValues {
 			if i >= 1 && callName == "println" {
 				b.createRuntimeCall("printspace", nil, "")
@@ -1746,6 +1747,7 @@ func (b *builder) createBuiltin(argTypes []types.Type, argValues []llvm.Value, c
 		if callName == "println" {
 			b.createRuntimeCall("printnl", nil, "")
 		}
+		b.createRuntimeCall("printunlock", nil, "")
 		return llvm.Value{}, nil // print() or println() returns void
 	case "real":
 		cplx := argValues[0]