Xabort on openbsd was panic(todo("")), so C abort(3) never terminated the
process by a signal. SQLite's crash-recovery tests (writecrash.test and the
crash*.test family) require the child to die BY A SIGNAL so the parent's Tcl
exec reports "child killed: ..."; the stub made it panic ("exited abnormally")
instead, which regressed with the SQLite 3.53.3 upgrade.

A faithful SIGABRT-based abort is unreachable from pure-Go libc on openbsd:
  - libc's own Xsigaction is an unimplemented stub, so SIGABRT's disposition
    cannot be reset to SIG_DFL; the Go runtime therefore always catches a
    delivered SIGABRT, prints a crash traceback, and exits status 2 — again
    "exited abnormally", not "killed".
  - openbsd's pinsyscalls(2) makes the raw sys_sigaction(46)/getthrid(299)/
    thrkill(119) syscalls return ENOSYS through the generic indirect syscall
    path (only libc.so's pinned call sites may issue them; verified: sigaction
    -> errno 78). So the netbsd-style SIG_DFL-reset + thread-directed thrkill
    approach cannot be ported.

Terminate via SIGKILL instead: it cannot be caught, blocked, or ignored, so it
kills the process immediately and unconditionally by signal — no race, no
traceback, no "SIGABRT" string leaking into test output. unix.Kill routes
through libc.so, so unlike the raw syscalls it is not defeated by pinsyscalls.

Fixes openbsd/amd64 and openbsd/arm64 (shared libc_openbsd.go). Validated
against SQLite 3.53.3: writecrash.test 0 errors out of 997 subtests, and the
whole crash*.test/multiplex/quota family green on both arches.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>