RFR: 8220741: ZGC: Move CPU agnostic files from linux_x86 to linux

[Per Liden]

The following files could be moved to src/hotspot/os/linux/gc/z since 
they are CPU agnostic:

src/hotspot/os_cpu/linux_x86/gc/z/zLargePages_linux_x86.cpp
src/hotspot/os_cpu/linux_x86/gc/z/zNUMA_linux_x86.cpp
src/hotspot/os_cpu/linux_x86/gc/z/zVirtualMemory_linux_x86.cpp


Bug: https://bugs.openjdk.java.net/browse/JDK-8220741
Webrev: http://cr.openjdk.java.net/~pliden/8220741/webrev.0

/Per