freebsd stable - Dec 2018 - FreeBSD-12 build question

1. How I can build release media of FreeBSD-12 on FreeBSD-11 system?
Currenly process failed by 'Abort trap'.

585191      121 -rw-------    1 root                             wheel          
8962048 Dec  5 18:58 ./ldconfig.core
585199      121 -rw-------    1 root                             wheel          
8953856 Dec  5 18:58 ./usr/obj/usr/src/amd64.amd64/mktemp.core
585200      249 -rw-------    1 root                             wheel          
9641984 Dec  5 18:58 ./usr/obj/usr/src/amd64.amd64/make.core

2. How I can update FreeBSD-11 (ZFS on Root) to FreeBSD-12 from source?
With new kernel ZFS not mounted, /bin/sh crashed.