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

[snake66]

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.

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Commit messages:
 - Add ucontext accessors for *BSD on Aarch64
 - Remove unused define in os_bsd_aarch64.cpp

Changes: https://git.openjdk.org/jdk/pull/25640/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25640&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358593
  Stats: 29 lines in 1 file changed: 27 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25640.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25640/head:pull/25640

PR: https://git.openjdk.org/jdk/pull/25640