RFR: 8245106: ZGC: Fix incorrect setup when using -XX:+UseTransparentHugePages

[Per Liden]

When using -XX:+UseTransparentHugePages, ZGC mistakenly calls 
madvise(..., MADV_HUGE); too late. The end result is that large pages 
are never allocated on first page fault, only via THP defrag. This in 
turn means that the majority of the heap will typically not be backed by 
large pages, which can have a very noticeable performance impact.

This patch depends on "8245098: Make SafeFetch32/N available earlier", 
which is currently out for review on hotspot-dev.

Bug: https://bugs.openjdk.java.net/browse/JDK-8245106
Webrev: http://cr.openjdk.java.net/~pliden/8245106/webrev.0

/Per