RFR (S) 8215521: add microbenchmark to measure AccessController.getContext

[eric.caspole at oracle.com]

Hi everybody,
Could I get some reviews for JDK-8215521 to add a new JMH for 
AccessController.getContext() -- this was written by Dean after a bug 
from almost a year ago, then we forgot about it. I made the stack depth 
into a parameter.

JBS:

https://bugs.openjdk.java.net/browse/JDK-8215521

Webrev:

http://cr.openjdk.java.net/~ecaspole/JDK-8215521/01/webrev/

Thanks,
Eric