NetBSD's Xabort was a panic(todo()) stub, so C abort(3) terminated the process
the wrong way: the Go runtime intercepts a delivered SIGABRT and exits with
status 2 instead of dying by signal. SQLite's crash tests (writecrash.test) fork
a child that abort()s and require it to be killed by SIGABRT; the stub made the
child "exit abnormally" instead, failing writecrash-1.1.1 on netbsd/amd64.

x/sys/unix exposes no high-level Sigaction on netbsd and Xsigaction is
unimplemented here, so reset SIGABRT to SIG_DFL via the raw
__sigaction_sigtramp(2) syscall (an all-zero struct sigaction = SIG_DFL;
trampoline/version are unused for SIG_DFL), then raise SIGABRT so the kernel
terminates the process by signal. ABI-preserving (Xabort signature unchanged).

Validated on NetBSD 10.1/amd64: a child calling libc.Xabort is now killed by
signal SIGABRT (exit 134), the behavior writecrash-1.1.1 expects.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>