Handle the version_script property.

Doesn't work when depends on arch/target/etc., but good enough for
libdl_android.

Bug: 186650430
Test: Presubmits.
Change-Id: Ib0facb41a89454717c74663e5e078aedd33d1b9c

bazel/properties.go

@@ -227,6 +227,15 @@ type Attribute interface {
 	HasConfigurableValues() bool
 }
 
+// Represents an attribute whose value is a single label
+type LabelAttribute struct {
+	Value Label
+}
+
+func (LabelAttribute) HasConfigurableValues() bool {
+	return false
+}
+
 // Arch-specific label_list typed Bazel attribute values. This should correspond
 // to the types of architectures supported for compilation in arch.go.
 type labelListArchValues struct {