RFR: 8237198+8237199+8237200: ZGC: Share heap multi-mapping code across platforms

[Per Liden]

Hi,

Please review this cleanup of the ZPhysicalMemory/ZBackingFile layer, 
which aims to de-duplicate some of the multi-mapping code. I've split 
the change into three separate patches, the main patch followed by two 
patches doing some renaming.


1) The ZBackingFile code was designed to allow platforms to decide if 
they want to use heap multi-mapping or some other (possibly HW 
supported) scheme. As of today, all our supported platforms do heap 
multi-mapping, so there's some degree of code duplication in 
ZBackingFile for each platform. This patch moves common multi-mapping 
code into ZPhysicalMemoryManager. If we in the future find that we want 
to support a platform that doesn't do multi-mapping, then we can 
introduce an abstraction for this again.
Bug: https://bugs.openjdk.java.net/browse/JDK-8237198
Webrev: http://cr.openjdk.java.net/~pliden/8237198/webrev.0


2) Rename ZBackingFile to ZPhysicalMemoryBacking, since "File" is 
somewhat misleading on platforms other than Linux.
Bug: https://bugs.openjdk.java.net/browse/JDK-8237199
Webrev: http://cr.openjdk.java.net/~pliden/8237199/webrev.0


3) Rename ZBackingPath to ZMountPoint, as it's a better name in light of 
JDK-8237199.
Bug: https://bugs.openjdk.java.net/browse/JDK-8237200
Webrev: http://cr.openjdk.java.net/~pliden/8237200/webrev.0


cheers,
Per