make universe fails with MAKEOBJDIRPREFIX

"make universe" consistently fails in buildworld when I set MAKEOBJDIRPREFIX.
I'm running head r306046 on amd64 and building head r306380.  There are only
comments in /etc/make.conf.  My complete command is:

    make universe JFLAG=-j16 MAKEOBJDIRPREFIX=/work/universe \
        > /usr/obj/build-logs/universe.log 2>&1 </dev/null &

There seem to be two distinct failures, depending on whether the target is cross
or native [enough].

On i386 and amd64, I get this:

======================================================================
--- all_subdir_rescue ---
1 warning generated.
--- cat.lo ---
cc -target x86_64-unknown-freebsd12.0 --sysroot=/work/universe2/usr/src/tmp -B/w
ork/universe2/usr/src/tmp/usr/bin -O2 -pipe   -std=gnu99    -Qunused-arguments
 -nostdlib -Wl,-dc -r -o cat.lo cat_stub.o /work/universe2/usr/src/rescue/rescue
//usr/src/bin/cat/cat.o
cc: error: no such file or directory: '/work/universe2/usr/src/rescue/rescue//us
r/src/bin/cat/cat.o'
--- all_subdir_lib ---
--- ktimer_getoverrun.po ---
--- all_subdir_rescue ---
*** [cat.lo] Error code 1

make[6]: stopped in /work/universe2/usr/src/rescue/rescue
1 error

make[6]: stopped in /work/universe2/usr/src/rescue/rescue
*** [rescue] Error code 2

make[5]: stopped in /usr/src/rescue/rescue
1 error
--- all_subdir_lib ---
======================================================================

On all other targets, I get this:

======================================================================
--- init_keytry.h ---
--- lib/libpam/libpam__L ---
--- openpam_strlcpy.pico ---
--- lib/ncurses/ncurses__L ---
./make_keys keys.list > init_keytry.h
--- lib/libpam/libpam__L ---
cc -isystem /work/universe/mips.mips/usr/src/tmp/usr/include -L/work/universe/mi
ps.mips/usr/src/tmp/usr/lib -B/work/universe/mips.mips/usr/src/tmp/usr/lib --sys
root=/work/universe/mips.mips/usr/src/tmp -B/work/universe/mips.mips/usr/src/tmp
/usr/bin -fpic -DPIC -g -O -pipe -I/usr/src/lib/libpam/libpam/../../../contrib/o
penpam/include -DLIB_MAJ=6 -DHAVE_DLFUNC=1 -DHAVE_FDLOPEN=1 -DHAVE_FPURGE=1 -DHA
VE_STRLCAT=1 -DHAVE_STRLCPY=1 -G0   -DOPENPAM_DEBUG -MD  -MF.depend.openpam_strl
cpy.pico -MTopenpam_strlcpy.pico -std=iso9899:1999 -Wsystem-headers -Werror -Wal
l -Wno-format-y2k -W -Wno-unused-parameter -Wstrict-prototypes -Wmissing-prototy
pes -Wpointer-arith -Wreturn-type -Wcast-qual -Wwrite-strings -Wswitch -Wshadow
-Wunused-parameter -Wcast-align -Wchar-subscripts -Winline -Wnested-externs -Wre
dundant-decls -Wold-style-definition -Wno-pointer-sign     -c /usr/src/lib/libpa
m/libpam/../../../contrib/openpam/lib/libpam/openpam_strlcpy.c -o openpam_strlcp
y.pico
--- lib/libthr__L ---
--- thr_detach.o ---
--- lib/ncurses/ncurses__L ---
sh: ./make_keys: Exec format error
*** [init_keytry.h] Error code 126

make[5]: stopped in /usr/src/lib/ncurses/ncurses
--- lib/libthr__L ---
cc -isystem /work/universe/mips.mips/usr/src/tmp/usr/include -L/work/universe/mi
ps.mips/usr/src/tmp/usr/lib -B/work/universe/mips.mips/usr/src/tmp/usr/lib --sys
root=/work/universe/mips.mips/usr/src/tmp -B/work/universe/mips.mips/usr/src/tmp
/usr/bin  -O -pipe -G0   -DPTHREAD_KERNEL -I/usr/src/lib/libthr/../libc/include
-I/usr/src/lib/libthr/thread  -I/usr/src/lib/libthr/../../include -I/usr/src/lib
/libthr/arch/mips/include -I/usr/src/lib/libthr/sys -I/usr/src/lib/libthr/../../
libexec/rtld-elf -I/usr/src/lib/libthr/../../libexec/rtld-elf/mips -I/usr/src/li
b/libthr/../libthread_db -Winline -fexceptions -D_PTHREAD_FORCED_UNWIND -D_PTHRE
ADS_INVARIANTS -MD  -MF.depend.thr_detach.o -MTthr_detach.o -std=gnu99 -Wsystem-
headers -Werror -Wall -Wno-format-y2k -W -Wno-unused-parameter -Wstrict-prototyp
es -Wmissing-prototypes -Wpointer-arith -Wno-uninitialized -Wno-pointer-sign
 -c /usr/src/lib/libthr/thread/thr_detach.c -o thr_detach.o
--- lib/ncurses/ncurses__L ---
1 error

make[5]: stopped in /usr/src/lib/ncurses/ncurses
*** [lib/ncurses/ncurses__L] Error code 2
======================================================================