RFR: 8212827: GlobalCounter should support nested critical sections

[Kim Barrett]

Please review this change to GlobalCounter to support nested critical
sections.

While in the area, cleaned up a few things:
- Improved comments for GlobalCounter API.
- Cleaned up write_synchronize global counter increment.
- Removed a useless acquire barrier in critical_section_begin.

CR:
https://bugs.openjdk.java.net/browse/JDK-8212827

Webrev:
http://cr.openjdk.java.net/~kbarrett/8212827/open.00/

Testing:
mach5 tier1-5
New gtest directly testing nested critical sections.