RFR: 8205064: Fail immediately if an unsupported GC is selected

[Per Liden]

If an unsupported GC (i.e. a GC that is not built into the VM) is 
selected by the user, the VM issues a warning and then continues and 
(silently) selects a different GC. Aleksey brought this up on the ZGC 
list [1]. I agree that this behavior seems dubious. With this patch we 
instead fail immediately to avoid unnecessary confusion.

Bug: https://bugs.openjdk.java.net/browse/JDK-8205064
Webrev: http://cr.openjdk.java.net/~pliden/8205064/webrev.0

Testing: manual + currently running in mach5 t{1,2,3}

/Per

[1] http://mail.openjdk.java.net/pipermail/zgc-dev/2018-June/000422.html