openjdk11: enable ZGC on x86_64-linux

The Z Garbage Collector is a concurrent, scalable, low latency garbage
collector designed to meet extremely-low-pause-time requirements for
small-to-multi-TB heap sizes.

ZGC can be enabled with the magical incantation:

    $ java -XX:+UnlockExperimentalVMOptions -XX:+UseZGC ...

Currently, ZGC is only available for x86_64-linux (though a port for
aarch64-linux may become available at a future time.) There are also a
number of other features that currently aren't present, such as JVMCI
integration (meaning compiler tools like Graal which require JVMCI will
not work with ZGC enabled.)

Signed-off-by: Austin Seipp <aseipp@pobox.com>