RFR: 8351159: Remaining cleanups in cpu/x86 after 32-bit x86 removal [v2]

[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

Anton Seoane Ampudia has updated the pull request incrementally with one additional commit since the last revision:

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27987/files
  - new: https://git.openjdk.org/jdk/pull/27987/files/a562af74..aa05c4a7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27987&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27987&range=00-01

  Stats: 14 lines in 6 files changed: 0 ins; 6 del; 8 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