RFR: 8222664: runtime/appcds/sharedStrings/SharedStringsStress.java assert GC active during NoGCVerifier

[Per Liden]

Deduplication of Strings must not happen when we're in a safepoint. 
However, during startup, we fail to block safepoint when calling 
deduplicate_shared_strings(), which sometimes results in hitting the 
NoSafepointVerifier assert.

Testing: tier1-3

Bug: https://bugs.openjdk.java.net/browse/JDK-8222664
Webrev: http://cr.openjdk.java.net/~pliden/8222664/webrev.0

/Per