RFR (S) 8216136: Don't take Compile_lock for SystemDictionary::_modification_counter

[coleen.phillimore at oracle.com]

Summary: make SystemDictionary::modification_counter atomic so not to 
require Compile_lock.

I moved updating the modification counter when the class is defined and 
added to the hierarchy.  I didn't remove the Compile_lock completely 
because there may be other code currently under the lock that needs it 
(flush_dependencies).   Can someone from the compiler area also review this?

Made Compile_lock an always safepointing lock.

Tested with mach5 tier1-6.

open webrev at http://cr.openjdk.java.net/~coleenp/8216136.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8216136

Thanks,
Coleen