RFR: 8237758: ZGC: Move get_mempolicy() syscall wrapper to ZSyscall

[Per Liden]

System call wrappers should live in ZSyscall, but the wrapper for 
get_mempolicy() currently lives in ZNUMA. We should move it.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237758
Webrev: http://cr.openjdk.java.net/~pliden/8237758/webrev.0

/Per