builtin: move C. calls to .c.v files (#11164)

vlib/builtin/option.c.v

@@ -0,0 +1,15 @@
+module builtin
+
+[typedef]
+struct C.IError {
+	_object voidptr
+}
+
+[unsafe]
+pub fn (ie &IError) free() {
+	unsafe {
+		ie.msg.free()
+		cie := &C.IError(ie)
+		free(cie._object)
+	}
+}