RFR: JDK-8221086: Shenandoah-specific workaround for JDK-8220671

[Roman Kennke]

JDK-8220671: Initialization race for non-JavaThread PtrQueue is making
troubles in Shenandoah's testing. While we're working out a generally
acceptable solution, we need a usable workaround in Shenandoah.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8221086
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8221086/webrev.01/

The proposed fix places a SuspendibleThreadJoiner around the
on_thread_attach() and on_thread_detach(), thereby preventing the race.

As soon as we figured out a solution for JDK-8220671 (which may well be
this 'workaround') we shall revert/overwrite this change.

Testing: many rounds of hotspot_gc_shenandoah and even more rounds of
the offending TestStringDedupStress.

Ok?

Roman