RFR: 8276055: ZGC: Defragment address space

[Per Liden]

In ZGC, small pages are allocated from the beginning of the address space (low addresses), while medium and large pages are allocated from the end of the address space (high addresses). However, small pages can sometimes end up on a high address if a medium or a large page was split into small pages. Over time this can cause the address space to become fragmented, with the risk of eventually running out of address space for new large allocations.

To mitigate this, we can defragment the address space by letting small pages at high addresses be remapped to lower addresses. That's what this patch does.

Testing:
 - Tier 1-7 with ZGC on Linux/x86_64
 - SPECjbb2015 shows no regression
 - Manuel ran test that tries to fragment the address space

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Commit messages:
 - 8276055: ZGC: Defragment address space

Changes: https://git.openjdk.java.net/jdk/pull/6137/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6137&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276055
  Stats: 69 lines in 7 files changed: 63 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6137.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6137/head:pull/6137

PR: https://git.openjdk.java.net/jdk/pull/6137