RFC: JEP JDK-8229358: ZGC: Support for macOS

[Erik Österlund]

Hi,

I prepared a new JEP [1], about adding support in ZGC for macOS. 
Implementing this is fairly straight forward now, after the work to have 
HotSpot deal with discontiguous heap reservations has completed. The 
reason this discontiguous heap reservation support was necessary is that 
macOS ASLR may put things in "our" address space, making the previously 
large and contiguous memory reservation fail. So now that the carpet has 
been rolled out, it will be a matter of using POSIX shared memory 
objects for ZGC bindings on macOS.

Feedback is more than welcome.

Thanks,
/Erik

[1] https://bugs.openjdk.java.net/browse/JDK-8229358