RFR (S) 8220153: Shenandoah does not work with TransparentHugePages properly

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8220153

Fix:
  http://cr.openjdk.java.net/~shade/8220153/webrev.02/

There are several troubles in current code. First, we need to make sure heap is aligned to make it
madvise'able by shared code during commits. Second, we have to pretouch with smaller pages when THP
is enabled, because we need to tell kernel all these smaller pages can be for coalesced into a huge
one. Third, when pretouching, it is apparently important to pretouch contiguous spaces separately (I
haven't verified why, but it is empirically better).

Unfortunately, constructing a reliable regression test proved hard, so I had to resort to manual
testing.

Testing: adhoc runs with THP enabled, hotspot_gc_shenandoah {with and without -XX:+UseTHP}

Thanks,
-Aleksey

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