RFR(XS): 8210259: [testbug] IncompatibleOptions.java fails if VM configured without ZGC

[Lindenmaier, Goetz]

Hi,

could I please get reviews for this tiny fix?
http://cr.openjdk.java.net/~goetz/wr18/8210259-fixNoZGCError/webrev.01/

The testcase enabling ZGC is protected by (Platform.isLinux() && Platform.isX64()). 
Thus, the test fails if the VM is compiled without ZGS on linuxx86_64.
Fix: use GC.Z.isSupported()

Best regards,
  Goetz.