runtime: crash on netbsd-arm64 when setcontext fails

Instead of exiting with status 16962 when we fail to call SYS_setcontext
in sigreturn, reference a null pointer and crash. Hopefully, this will
enable grabbing a core dump to debug.

Updates #42422

Change-Id: If02c14a0a37084351f3f00db3dc9766cb68ae4b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/375834


Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Benny Siegert <bsiegert@gmail.com>