RFR: 8354853: Clean up x86 registers after 32-bit x86 removal

[Manuel Hässig]

This PR cleans up some 32-bit remnants in the x86 register code. This also presented the opportunity to convert the unscoped enums into typed constants.

Testing:
 - [ ] Github Actions
 - [ ] tier1,tier2 on linux-x64-debug, linux-x64, windows-x64-debug, windows-x64, macosx-x64-debug, macosx-x64

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Commit messages:
 - Clean up x86 registers aftr 32-bit removal

Changes: https://git.openjdk.org/jdk/pull/29224/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29224&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354853
  Stats: 62 lines in 5 files changed: 0 ins; 39 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/29224.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29224/head:pull/29224

PR: https://git.openjdk.org/jdk/pull/29224