Add option for Implementation JEP 312: Thread-local handshake

Summary

Introduce an option to control use of the new Thread-local handshake safepoint mechanism introduced by JEP 312.

Problem

JEP 312 introduces a new safepoint mechanism: Thread-local handshakes. To opt-out on supported platforms of using this new mechanism a new VM flag is required.

Solution

Add a new product option ThreadLocalHandshakes (default true) to allow users to select the new safepoint mechanism.

Specification

Complete code is at: http://cr.openjdk.java.net/~rehn/8185640/

+  product_pd(bool, ThreadLocalHandshakes,                                   \
+          "Use thread-local polls instead of global poll for safepoints.")  \
+          constraint(ThreadLocalHandshakesConstraintFunc,AfterErgo)         \
+                                                                            \