RFR: 8214455: Relocate CDS archived regions to the top of the G1 heap

[Ioi Lam]

When the runtime heap and dump time heap have different sizes, it's possible that the archived heap regions are mapped to the middle of the runtime heap. Because the archived heap regions are pinned, this could reduce the size of the largest "humongous" array allocation. In the worst case, the maximum allocatable array length may be half of the optimal value.

The fix is to relocate the archived regions to the top of the  G1 heap (currently archived regions are supported only by G1 on 64-bit JVM).

Note that usually the top of the G1 heap is placed just below the 32GB boundary. As a result, the archived heap regions are at the same location between run time and dump time, so no relocation is necessary.

In mach5 testing, we occasionally run into this problem (see https://bugs.openjdk.java.net/browse/JDK-8239089), probably because we fail to reserve the heap below 32GB due to address space layout randomization (ASLR).

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Commit messages:
 - Added -Xlog:cds=debug in case this test fails again
 - 8214455: Relocate CDS archived regions to the top of the G1 heap

Changes: https://git.openjdk.java.net/jdk/pull/3349/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3349&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8214455
  Stats: 143 lines in 8 files changed: 139 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3349.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3349/head:pull/3349

PR: https://git.openjdk.java.net/jdk/pull/3349