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

[Kim Barrett]

On Wed, 26 Mar 2025 12:48:13 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> 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`
>  - [ ] Linux x86_64 server fastdebug, `all`

Looks good.

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Marked as reviewed by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24253#pullrequestreview-2718385753