RFR: 8156871: Possible concurrency issue with JVM_AddModuleExports

[Lois Foltan]

Hello,

Please review the work to finalize a concurrency issue when setting the 
exported state of a PackageEntry.  The work completed in bug 
https://bugs.openjdk.java.net/browse/JDK-8152404 actually had the side 
effect of fixing this bug as well.  Prior to JDK-8152404, each 
PackageEntry determined its exported state via two flags, one of which 
was a general _is_exported flag followed by a another more specific 
state of exportability flag that determined if the package was exported 
qualifiedly or not to a given module.  Checking these two flags, as the 
PackageEntry::is_* methods used to do without the Module_lock, was 
problematic and yielded a situation where a call to add a module to a 
PackageEntry's qualified exported entry list failed because it was 
determined that the package was unqualifiedly exported when it really 
was not.

To complete this fix, I have removed the is_unqual_exported call prior 
to setting a PackageEntry's exported list.  The method 
PackageEntry::set_exported determines what the current package export 
state is and acts correctly.  Also, the test case has been added in this 
webrev since it is a good stress test case for JVM module support.

Passes JPRT, java/lang, java/util RBT hotspot nightly.

Bug: https://bugs.openjdk.java.net/browse/JDK-8156871
Open webrev: http://cr.openjdk.java.net/~lfoltan/bug_jdk8156871/webrev/

Thanks,
Lois