rust: Refactor staticStd to stdLinkage

Instead of returning a boolean, return an enum value to improve
readability and provide greater flexibility for future modifications.

Bug: 168729404
Test: Soong tests pass
Change-Id: Iddcdae8c34be09e476404382e43d1ea5935bae65

rust/test.go

@@ -134,6 +134,6 @@ func RustTestHostFactory() android.Module {
 	return module.Init()
 }
 
-func (test *testDecorator) staticStd(ctx *depsContext) bool {
-	return true
+func (test *testDecorator) stdLinkage(ctx *depsContext) RustLinkage {
+	return RlibLinkage
 }