8062247: [TESTBUG] Allow WhiteBox test to access JVM offsets

Reviewed-by: coleenp, iklam, mseledtsov
Contributed-by: yumin.qi@oracle.com

test/testlibrary/whitebox/sun/hotspot/WhiteBox.java

@@ -208,4 +208,13 @@ public class WhiteBox {
                        .findAny()
                        .orElse(null);
   }
+  public native int getOffsetForName0(String name);
+  public int getOffsetForName(String name) throws Exception {
+    int offset = getOffsetForName0(name);
+    if (offset == -1) {
+      throw new RuntimeException(name + " not found");
+    }
+    return offset;
+  }
+
 }