RFR: 8342504: Remove NMT header and footer canaries

[Johan Sjölen]

Today NMT has two canaries: A header and a footer canary. These enable mainly two things:

1. For NMT to aid in describing a pointer
2. A basic form of out-of-bounds protection

With the introduction of UBSan and Asan into OpenJDK we have gained stronger tools for this sort of analysis, without requiring NMT to be activated. Therefore, I believe that point 2 is no longer something that NMT needs to support. For point number one, we will unfortunately be losing this ability.

I want to delete these canaries to open up a few free bytes. These can allow us to have "practically unlimited" (4 bytes) of memory tags.

tier1-tier2 tests succeeded.

I am awaiting discussion on the Hotspot-dev mailing list, but keeping this PR open for review.

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Commit messages:
 - Remove canaries

Changes: https://git.openjdk.org/jdk/pull/21560/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21560&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342504
  Stats: 361 lines in 8 files changed: 0 ins; 349 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/21560.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21560/head:pull/21560

PR: https://git.openjdk.org/jdk/pull/21560