runtime: crash on netbsd-arm64 when setcontext fails
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 <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Trust: Benny Siegert <05f4aee36fd25d0de2a07789fd6e73398db492cc@gmail.com>

golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,golang/go,CAFxX/go,golang/go,cloudflare/go