RFR: JDK-8248725: Shenandoah: Claim verifier thread roots for parallel processing

[rkennke at redhat.com]

We see occasional crashes during CI testing:

# Internal Error (/home/jenkins/workspace/nightly/jdk-
jdk/src/hotspot/cpu/x86/frame_x86.cpp:684), pid=25982, tid=26009
# assert(_last_Java_pc == NULL) failed: already walkable

This seems to be caused by calls to
Threads::possibly_parallel_oops_do() with par=false even though it's
called by multiple workers. This leads to threads being visited by more
than one worker, and thus trip this assert.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8248725
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8248725/

Testing: multiple runs of hotspot_gc_shenandoah (which exposed the bug
before)

Is the right way to push this to: http://hg.openjdk.java.net/jdk/jdk15/
 ?

Roman