RFR: 8212748: ZGC: Add reentrant locking functionality

[Erik Österlund]

Hi,

ZGC needs a per-nmethod lock to be used for concurrent IC cleaning, 
protecting misaligned oops from concurrently being patched by nmethod 
entry barriers, and read using CompiledMethod::is_unloading(), with 
interactions crossing JavaThreads and non-Java threads. This patch adds 
that utility.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8212748/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8212748

Thanks,
/Erik