Add setup for GraalVM native-image

[koppor]

Follow-up to #212

veles needs GraalVM native-image, which seems to be a bit complicated to get it running: https://www.graalvm.org/latest/reference-manual/native-image/

Would a support for this in-scope or out-of-scope for gg.cmd? ^^

[eirikb]

Doing some housekeeping in old discussions.

This is done now, out in 205. gg.cmd java@21-graal gives you GraalVM CE, and native-image is bundled so there is no setup step. GRAALVM_HOME gets set too.

More details over in #284.

Closing this one