Code review request: 8005936,PrintNMTStatistics doesn't work for normal JVM exit

[Zhengyu Gu]

Moved NMT shutdown code to JVM exit handler, so guarantees that NMT 
memory statistics can be reported when PrintNMTStatistics flag is enabled.

webrev: http://cr.openjdk.java.net/~zgu/8005936/webrev.00/

Tests:

1. JPRT tests
2. UTE vm.quick.testlist on Windows x64, Solaris sparv9, Linux x32 and x64


Thanks,

-Zhengyu