RFR: 8356289: Shenandoah: Clean up SATB barrier runtime entry points

[William Kemper]

On Tue, 19 Aug 2025 23:47:34 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

> The runtime entry points for the SATB slow-paths currently take a JavaThread* argument. When @rkennke did the entry points for Graal ([JDK-8356075](https://bugs.openjdk.org/browse/JDK-8356075)), he figured that we do not really need them. The slow-path is only called rarely (whenever the local SATB buffer is full), and getting the current Thread* in the runtime would not be the expensive part.
> 
> This PR is a patch to do that clean up. The changes were tested on Linux x64/aarch64 with JTREG tier1-3 using Shenandoah for all tests.

Internal testing looks good.

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Marked as reviewed by wkemper (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26850#pullrequestreview-3146240854