RFR: 8237649: ZGC: Improved NUMA support when using small pages

[Per Liden]

The NUMA allocation support in ZGC works as expected only when using 
-XX:+UseLargePages. The reason is that, on Linux, small pages are 
allocated at commit/fallocate time and is controlled by the NUMA policy 
of the current thread, while large pages are allocated at page fault 
time and is controlled by the NUMA policy of the memory range. ZGC 
currently only sets up the NUMA policy for the memory range, which has 
no effect on small pages (since they are allocated by tmpfs rather than 
being anonymous mappings).

We should fix this, so that the NUMA allocation support works equally 
well for small pages.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237649
Webrev: http://cr.openjdk.java.net/~pliden/8237649/webrev.0

/Per