ZGC support

[vans239]

It seems that ZGC doesn't support EnableJVMCI currently and will not support in nearest future

Env:

• jave 20.0.2
• -XX:+UseZGC

Setting EnableJVMCI to false as selected GC does not support JVMCI: z gc

It also seems like the support was added for non-generational zgc in java 21
Java 20: bool JVMCIGlobals::gc_supports_jvmci() { return UseSerialGC || UseParallelGC || UseG1GC; }
Java 21: bool JVMCIGlobals::gc_supports_jvmci() { return UseSerialGC || UseParallelGC || UseG1GC || (UseZGC && !ZGenerational); }

Are you aware of any workarounds?
Would it be worth to outline in docs that some gcs don't EnableJVMCI?

[apangin]

will not support in nearest future
...
was added for non-generational zgc in java 21

JDK 21 is about a month ahead from today - pretty much nearest future :)
I have no plans to make nalim work in configurations where JVMCI is not supported.

Would it be worth to outline in docs that some gcs don't EnableJVMCI?

Added a note in README.