RFR: 8266787: Potential overflow of pointer arithmetic in G1ArchiveAllocator

[Albert Mingkun Yang]

Identified during the review of #3900. Using `pointer_delta` to fix the issue and some general cleanup.

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Commit messages:
 - overflow

Changes: https://git.openjdk.java.net/jdk/pull/3936/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3936&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266787
  Stats: 17 lines in 1 file changed: 5 ins; 1 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3936.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3936/head:pull/3936

PR: https://git.openjdk.java.net/jdk/pull/3936