RFR: 8351157: Clean up x86 GC barriers after 32-bit x86 removal [v2]

[Aleksey Shipilev]

> Assembler GC barriers have quite a bit of coding to support 32-bit x86. As 32-bit x86 is removed, we can clean up those parts.
> 
> We can eliminate `!LP64` blocks quite easily. We can also prune passing around `thread` argument, and just trust that `r15_thread` is always available.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `tier1`
>  - [x] Linux x86_64 server fastdebug, `all`

Aleksey Shipilev 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 branch 'master' into JDK-8351157-x86-gc-barriers
 - Also do tlab_allocate
 - Rely on R15 to be a thread register
 - Work

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Changes: https://git.openjdk.org/jdk/pull/24253/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24253&range=01
  Stats: 543 lines in 20 files changed: 1 ins; 426 del; 116 mod
  Patch: https://git.openjdk.org/jdk/pull/24253.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24253/head:pull/24253

PR: https://git.openjdk.org/jdk/pull/24253