Convert $(sort) to Starlark

This is mostly so that we can not error out when
seeing sort inside a `include $(sort $(wildcard foo/*.mk))`.

Bug: 218736658
Test: go test
Change-Id: I827ecfc751a908227cb2dd9ed1e4b3f313791a16
diff --git a/mk2rbc/mk2rbc.go b/mk2rbc/mk2rbc.go
index 8807437..5ce031d 100644
--- a/mk2rbc/mk2rbc.go
+++ b/mk2rbc/mk2rbc.go
@@ -110,6 +110,7 @@
 	"require-artifacts-in-path-relaxed":    &simpleCallParser{name: baseName + ".require_artifacts_in_path_relaxed", returnType: starlarkTypeVoid},
 	// TODO(asmundak): remove it once all calls are removed from configuration makefiles. see b/183161002
 	"shell":    &shellCallParser{},
+	"sort":     &simpleCallParser{name: baseName + ".mksort", returnType: starlarkTypeList},
 	"strip":    &simpleCallParser{name: baseName + ".mkstrip", returnType: starlarkTypeString},
 	"subst":    &substCallParser{fname: "subst"},
 	"warning":  &makeControlFuncParser{name: baseName + ".mkwarning"},