RFR: 8342504: Remove NMT header and footer canaries [v2]

[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 incrementally with one additional commit since the last revision:

  Rename flags to tags

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21560/files
  - new: https://git.openjdk.org/jdk/pull/21560/files/90734f6a..838b80d2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21560&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21560&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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