RFR: 8233281: Obsolete UseSemaphoreGCThreadsSynchronization

[Stefan Karlsson]

Hi all,

Please review this patch to obsolete the diagnostic 
UseSemaphoreGCThreadsSynchronization JVM option.

https://bugs.openjdk.java.net/browse/JDK-8233281
https://cr.openjdk.java.net/~stefank/8233281/webrev.01

I added this flag when implementing:
8087324: Use semaphores when starting and stopping GC task threads

It was intended to be a fallback solution if the new semaphore 
implementation failed to to work for any reason. We've used the 
semaphore code for many years now, and the Mutex implementation has bit 
rottened.

The proposed patch both obsoletes the the JVM flag, and removes the 
Mutex dispatch mechanism.

Thanks,
StefanK