RFR: 8342504: Remove NMT header and footer canaries [v3]

[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.

Johan Sjölen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge remote-tracking branch 'openjdk/master' into delete-canaries
 - Comment
 - Rename flags to tags
 - Remove canaries

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Changes: https://git.openjdk.org/jdk/pull/21560/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21560&range=02
  Stats: 366 lines in 8 files changed: 0 ins; 353 del; 13 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