[Issue 22545] New: OpenBSD: Always use system backtrace

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22545

          Issue ID: 22545
           Summary: OpenBSD: Always use system backtrace
           Product: D
           Version: D2
          Hardware: All
                OS: Other
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: bcallah at openbsd.org

This PR unconditionally uses the system backtrace facilities on OpenBSD rather
than using a hand-rolled version when using !GDC.

Discovered when building LDC on OpenBSD/AArch64, which failed because the
hand-rolled version doesn't understand AArch64.

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