RFR: 8230090: ZGC: Introduce ZSyscall

[Per Liden]

Move the raw syscalls done in ZBackingFile into a class that can be 
shared by all CPUs on Linux. Only the SYS_<number> macros need to be CPU 
specific. This is paves the way for JDK-8230092, where I consolidate 
ZBackingFile, ZBackingPath and ZPhysicalMemoryBacking, which today are 
duplicated on linux_x86 and linux_aarch64.

Bug: https://bugs.openjdk.java.net/browse/JDK-8230090
Webrev: http://cr.openjdk.java.net/~pliden/8230090/webrev.0

Testing: Builds on linux_x86 and linux_aarch64, passes dacapo

/Per