RFR: 8358593: Add ucontext accessors for *BSD on Aarch64

[duke]

On Wed, 4 Jun 2025 12:58:57 GMT, snake66 <duke at openjdk.org> wrote:

> This adds macros for non-OSX BSD variants to access the required fields from ucontext_t in os_bsd_aarch64.cpp, similar to how it's already done for x86_64 in os_bsd_x86.cpp.

@snake66 
Your change (at version c189e74c9bebc4f661572576e2f4d8d1035d8f76) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/jdk/pull/25640#issuecomment-2943539159