RFR: 8351159: Remaining cleanups in cpu/x86 after 32-bit x86 removal

[Anton Seoane Ampudia]

This PR carries out cleanup on the `cpu` and `os_cpu` directories after the removal of the 32-bit x86 architecture from HotSpot.

Mainly, code guarded by `IA32` or `X32`, or `!AMD64` or `!LP64` in a x86 context, has been removed. These guards are now redundant (e.g. checking for 64bit on a x86 context) or the code blocks never executed (e.g. code under `#ifdef X32`). Additionally, x86 AD files have been merged.

Please note that this changeset limits itself to changes under `cpu/x86` and `os_cpu/x86`. Other cleanups are filed under a different RFE, and consequently addressed in a different PR: [JDK-8351149](https://github.com/openjdk/jdk/pull/27990).

**Testing:** passes tiers 1-5

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Commit messages:
 - Update copyright years
 - Reintroduce changes from JDK-8369569
 - Merge upstream changes
 - Merge x86 AD files
 - Roll back changes that are targeted in JDK-8354853
 - Cleanup _M_AMD64 from cpu/ and os_cpu/
 - Cleanup LP64_ONLY and NOT_LP64 from cpu/ and os_cpu/
 - Cleanup LP64 from os_cpu/
 - Cleanup LP64 from cpu/
 - Cleanup AMD64_ONLY and NO_AMD64 from cpu/ and os_cpu/
 - ... and 3 more: https://git.openjdk.org/jdk/compare/470eedb1...a562af74

Changes: https://git.openjdk.org/jdk/pull/27987/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27987&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351159
  Stats: 32659 lines in 32 files changed: 15343 ins; 16314 del; 1002 mod
  Patch: https://git.openjdk.org/jdk/pull/27987.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27987/head:pull/27987

PR: https://git.openjdk.org/jdk/pull/27987