Disable "__builtin_func" when converting mk to bp

The "__builtin_func" doesn't work in soong. The converting tool
androidmk should disable this feature.

Bug: 329770862
Test: 1. cd build/soong/androidmk/androidmk
      2. go test -run TestEndToEnd
Change-Id: I1de04e4e1a34fb7335e0013f59b33f7f2673722a

androidmk/parser/scope.go

@@ -14,9 +14,7 @@
 
 package parser
 
-import (
-	"strings"
-)
+import "strings"
 
 type Scope interface {
 	Get(name string) string
@@ -88,7 +86,7 @@ func (v Variable) EvalFunction(scope Scope) ([]string, bool) {
 			if fname == "call" {
 				return scope.Call(argVals[0], argVals[1:]), true
 			} else {
-				return []string{"__builtin_func:" + fname + " " + strings.Join(argVals, " ")}, true
+				return []string{"UNSUPPORTED FUNCTION:" + fname + " " + strings.Join(argVals, " ")}, true
 			}
 		}
 	}