RFR (S) 5103339: Strengthen NoSafepointVerifier

[coleen.phillimore at oracle.com]

Summary: Add NSV check at possible safepoint transition or places that 
could take out locks.  Consolidate with clearing unhandled oops.

This change checks for NoSafepointVerifier no_safepoint_counts at 
possible safepoints.  The starting set is at transitions, and in the 
"else" clauses where CHECK_UNHANDLED_OOPS were clearing unhandled oops.  
Some of these were removed because they weren't places with possible 
safepoints, so were wrong.

The unhandled oops clearing and no_safepoint counter check are now done 
in the same function.  MemAllocator -> check_for_valid_allocation_state 
calls check_for_valid_safepoint_state which calls check_possible_safepoint.

Calls to check_possible_safepoint are in DEBUG_ONLY when 
Thread::current() is called.

I had to remove it from the else clause in JvmtiThreadState because it's 
called from a place that cannot safepoint (see vtableStubs.cpp).

os.cpp ResourceMark needed for debugging.

Tested with tier1 on all Oracle platforms, and tier 1-3 on linux-x64-debug.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/5103339.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-5103339

Thanks,
Coleen