r312349 crashes: null filesizes: rescue with usb stick impossible due to lack of CC

FreeBSD crashes (r312349): during installworld and installkernel, the
running system crashed and left me with a nullyfied Samsung 850 PRO
SSD, means: almost every binary in /bin, /sbin, /usr/bin, /usr/sbin has
filesize NULL, execpt /bin/sh. Booting is impossible.

Since it is unknown what the crash triggered - I just tried to rescue
my box from crashing by adding this faulty EARLY_AP_STARTUP option to
my customized kernel - I intend to rescue the system by installing the
binaries again via make installworld by using an USB image from the
FreeBSD site:

 FreeBSD-12.0-CURRENT-amd64-20170105-r311461-memstick.img

Booting this USB image, mounting all of the filesystems of the SSD
of the trunkated system into /mnt
(/mnt/usr, /mnt/usr/local etc) execpt /usr/src and /usr/obj, which
Imount to the USB's /usr/src and /usr/ob, and then trying to perform the
installworld again via

cd /usr/src
make DESTDIR=/mnt  installkernel installworld

I fail: I'm bothered with the lack of a compilerand enforced to set
COMÜILER_TYPE, but this is useless, since this minimalistic and useless
provided USB (and CD image) does obviusly not contain any compiler.

So, I'm stranded!

I found several websites on which exact such a rescue procedure is
explained and I remember that I rescued the same way last year an
11-CURRENT system. At this moment, I can not fathom what kind of mind
is behind of the reduction of the images and extracting essential parts.

How am I supposed to rescue a system the way I try to? I have an
intact /usr/obj, /usr/src, I have this crap USB image wihich is
obviously incapable of performing such kind of rescue. The system in
question does not have a DVD or CD drive, it is USB only. 

Any help appreciated.

Please point me to the proper webpage on the FBSD site on which it
might be to find how to rescue a system like I'm inclined to (if ever).
I couldn't find any suitable notices.

many thanks in advance,

Oliver