Fix +build tag logic

core/runtime/os_specific_freestanding.odin

@@ -0,0 +1,21 @@
+//+build freestanding
+package runtime
+
+_OS_Errno  :: distinct int;
+_OS_Handle :: distinct uintptr;
+
+os_stdout :: proc "contextless" () -> _OS_Handle {
+	return 1;
+}
+os_stderr :: proc "contextless" () -> _OS_Handle {
+	return 2;
+}
+
+// TODO(bill): reimplement `os.write`
+os_write :: proc(fd: _OS_Handle, data: []byte) -> (int, _OS_Errno) {
+	return 0, -1;
+}
+
+current_thread_id :: proc "contextless" () -> int {
+	return 0;
+}