RFR (S) 8217378: UseCriticalCMSThreadPriority is broken

[Roman Kennke]

Looks good to me.

Thanks,
Roman

> Bug:
>   https://bugs.openjdk.java.net/browse/JDK-8217378
> 
> Fix:
>   http://cr.openjdk.java.net/~shade/8217378/webrev.01/
> 
> While this concerns the experimental CMS option, the fix itself also unblocks Shenandoah RFE
> (JDK-8217343). There might be a larger discussion if CriticalPriority thingie is useful, but I would
> first push the simple fix that makes it work, at least to have the up-to-date performance data. It
> should make the most benefit on Solaris where critical prio can map to FX priorities, but
> non-Solaris builds can also enjoy the (control) GC threads priority elevated to MaxPriority, above
> the VMThread.
> 
> Testing: new test, hotspot tier1, jdk-submit (running)
> 
> Thanks,
> -Aleksey
>