Make +UseZGC fail hard when it is unsupported

[Aleksey Shipilev]

Hey guys,

I have build recent workspace, and forgot to enable zgc in configure. Yet, the runs proceeded as
usual, and G1 was automatically selected. Per, can you make this failure fatal?

$ build/linux-x86_64-normal-server-release/images/jdk/bin/java -XX:+UnlockExperimentalVMOptions
-XX:+UseZGC -Xlog:gc -version
OpenJDK 64-Bit Server VM warning: -XX:+UseZGC not supported in this VM
[0.015s][info][gc] Using G1
<proceeds as usual>

Thanks,
-Aleksey