RFR (trivial) 8209541: Fix merge problem in SymbolTable::do_check_concurrent_work

[coleen.phillimore at oracle.com]

Summary: remerged with previous change to do_unloading.

I also moved the call to SystemDictionary::do_unloading() for some 
future change for concurrent class unloading.

open webrev at http://cr.openjdk.java.net/~coleenp/8209541.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8209541

Tested with hs-tier1,2.

Thanks,
Coleen