Running 8 building 10 gives "Out of file descriptors" ?

[Dieter BSD]

amd64 running 8.2
Attempting to update to 10.1
Need to build custom 10.1 kernel on machine running 8.2.
Installed binaries and src off to the side.
Attempts to compile kernel directly failed.
Attempts to build 10.1 "make" as 8.2 binary failed.
Chroot to 10.1 tree.
Attempt to build kernel (using stock GENERIC config file and stock sources
for now) failed:

>>> stage 3.1: making dependencies
--------------------------------------------------------------
cd /usr/obj/usr/src/sys/GENERIC; MAKEOBJDIRPREFIX=/usr/obj  MACHINE_ARCH=amd64 \
 MACHINE=amd64  CPUTYPE= GROFF_BIN_PATH=/usr/obj/usr/src/tmp/legacy/usr/bin  GR\
OFF_FONT_PATH=/usr/obj/usr/src/tmp/legacy/usr/share/groff_font  GROFF_TMAC_PATH\
=/usr/obj/usr/src/tmp/legacy/usr/share/tmac  _SHLIBDIRPREFIX=/usr/obj/usr/src/t\
mp  _LDSCRIPTROOT=  VERSION="FreeBSD 10.1-RELEASE amd64 1001000"  INSTALL="sh /\
usr/src/tools/install.sh"  PATH=/usr/obj/usr/src/tmp/legacy/usr/sbin:/usr/obj/u\
sr/src/tmp/legacy/usr/bin:/usr/obj/usr/src/tmp/legacy/usr/games:/usr/obj/usr/sr\
c/tmp/legacy/bin:/usr/obj/usr/src/tmp/usr/sbin:/usr/obj/usr/src/tmp/usr/bin:/us\
r/obj/usr/src/tmp/usr/games:/sbin:/bin:/usr/sbin:/usr/bin CC="cc " CXX="c++ "  \
CPP="cpp "  AS="as" AR="ar" LD="ld" NM=nm  OBJDUMP= RANLIB=ranlib STRINGS= COMP\
ILER_TYPE=clang make  -m /usr/src/share/mk  KERNEL=kernel depend -DNO_MODULES_O\
BJ
machine -> /usr/src/sys/amd64/include
x86 -> /usr/src/sys/x86/include
cc  -c -O2 -pipe -fno-strict-aliasing -std=c99 -g -Wall -Wredundant-decls -Wnes\
ted-externs -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Winline -\
Wcast-qual -Wundef -Wno-pointer-sign -fformat-extensions -Wmissing-include-dirs\
 -fdiagnostics-show-option -Wno-error-tautological-compare -Wno-error-empty-bod\
y -Wno-error-parentheses-equality -Wno-error-unused-function -nostdinc -I. -I/u\
sr/src/sys -I/usr/src/sys/contrib/altq -I/usr/src/sys/contrib/ipfilter -I/usr/s\
rc/sys/dev/ath -I/usr/src/sys/dev/ath/ath_hal -I/usr/src/sys/contrib/dev/ath/at\
h_hal -I/usr/src/sys/contrib/ngatm -I/usr/src/sys/dev/twa -I/usr/src/sys/dev/cx\
gb -I/usr/src/sys/dev/cxgbe -I/usr/src/sys/contrib/libfdt -D_KERNEL -DHAVE_KERN\
EL_OPTION_HEADERS -include opt_global.h -fno-omit-frame-pointer -mno-omit-leaf-\
frame-pointer -mno-aes -mno-avx -mcmodel=kernel -mno-red-zone -mno-mmx -mno-sse\
 -msoft-float -fno-asynchronous-unwind-tables -ffreestanding -fstack-protector \
-gdwarf-2 /usr/src/sys/amd64/amd64/genassym.c
NM='nm' sh /usr/src/sys/kern/genassym.sh genassym.o > assym.s
Out of file descriptors
*** Error code 2


Maybe I need to build the 10.1 toolchain as 8.2 binaries?
nice -20 make kernel-toolchain > /tmp/make_kernel_toolchain_out 2>&1
Result:
mkdep -f .depend -a    -I/usr/src/tools/build/../../contrib/libc-pwcache -I/usr\
/src/tools/build/../../lib/libc/include -I/usr/obj/usr/src/tmp/legacy/usr/inclu\
de -std=gnu99   /usr/src/tools/build/../../contrib/libc-pwcache/pwcache.c
Out of file descriptors

Wrote a very small C program.  Result:
getdtablesize() returned 11095

The 10.1 mkdep is the same as 8.2 except for comments.

It seems unlikely that it really needs more than 11095 file descriptors.
So what is going on, and how do I fix it?

[ Obviously I have thought of booting stock 10.1 and then building
new kernel, but I can't take the machine down that long. ]