RFR: 8224817: Implementation of JEP 364: ZGC on macOS

[erik.osterlund at oracle.com]

Hi,

Now that some curling has been performed, paving way for this patch:

     8229027: Improve how JNIHandleBlock::oops_do distinguishes oops 
from non-oops
     8229278: Improve hs_err location printing to assume less about GC 
internals
     8229189: Improve JFR leak profiler tracing to deal with 
discontiguous heaps
     8224815: Remove non-GC uses of CollectedHeap::is_in_reserved()
     8224820: ZGC: Support discontiguous heap reservations

...the remaining thing to do is plugging in a few platform specific ZGC 
files. This patch does that.
Decided to go with mach_vm_map/mach_vm_remap to implement multi-mapping. 
Previously I didn't want to do that as I couldn't figure out how to 
mach_vm_remap memory on top of reserved VA (acquired using mmap). But 
apparently VM_FLAGS_OVERWRITE was the missing ingredient there. With 
that in place, dodging the terrible ftruncate implementation on macOS 
seemed like a good idea. That also implies this port supports large 
pages (unlike other GCs on macOS today). Yay!

CR:
http://cr.openjdk.java.net/~eosterlund/8224817/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8229358

Thanks,
/Erik