Reorganize package runtime

Separates out the OS specific stuff into different files

core/runtime/defaults.odin

@@ -0,0 +1,21 @@
+package runtime
+
+import "core:os"
+
+current_thread_id :: proc "contextless" () -> int {
+	return os.current_thread_id();
+}
+
+default_assertion_failure_proc :: proc(prefix, message: string, loc: Source_Code_Location) {
+	fd := os.stderr;
+	print_caller_location(fd, loc);
+	os.write_string(fd, " ");
+	os.write_string(fd, prefix);
+	if len(message) > 0 {
+		os.write_string(fd, ": ");
+		os.write_string(fd, message);
+	}
+	os.write_byte(fd, '\n');
+	debug_trap();
+}
+