RFR 8227104: Shenandoah: Implementation of ShenandoahReentrantLock

[Zhengyu Gu]

This is a prerequisite for concurrent nmethod unloading [1]. Currently, 
it has no users, but will be embedded into ShenandoahNMethod.

I separate the two changes, in case we may want to backport 
ShenandoahNMethod changes.

Bug: https://bugs.openjdk.java.net/browse/JDK-8227104
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8227104/webrev.00/

Test:
   fastdebug and release builds.

Thanks,

-Zhengyu

[1] https://bugs.openjdk.java.net/browse/JDK-8227102