RFR: 8285984: G1: Use standard idiom for inlined payload in G1MonotonicArena::Segment

[Leo Korinth]

This change remove the old padding (extremely big, especially on x86-64), and replaces it with a guaranteed 8 byte alignment. It also removes the `_bottom` field and instead adds a `payload` method. I statically assert that Segment is 8 byte aligned. I then runtime assert that Segment is constructed on 8 byte aligned memory. I also assert that the Slot alignment is less than or equal to the Segment alignment.

I am running tier 1-3 at the moment. Do you think I should run some specific performance test on this?

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Commit messages:
 - 8285984: G1: Use standard idiom for inlined payload in G1MonotonicArena::Segment

Changes: https://git.openjdk.org/jdk/pull/27258/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27258&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8285984
  Stats: 19 lines in 3 files changed: 4 ins; 7 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/27258.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27258/head:pull/27258

PR: https://git.openjdk.org/jdk/pull/27258