RFR: 8237825: ZGC: Replace -XX:ZPath with -XX:AllocateHeapAt

[Per Liden]

ZGC has the option -XX:ZPath to allow a user to explicitly specify where 
the backing file system is located. However, after ZPath was introduced, 
a new generic option -XX:AllocateHeapAt was introduced. This option is 
used the other GCs and have the same meaning/purpose and it's arguably 
better named. There's no good reason why ZGC shouldn't use that too, 
instead of -XX:ZPath.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237825
Webrev: http://cr.openjdk.java.net/~pliden/8237825/webrev.0

/Per