installworld failed by mounting other's /usr/obj

I have one machine which is faster to build world and export /usr/obj for
others to install.

As of r297266 installworld would fail. It seems to be caused by r296921
which would delete libc.ld. In this case libc.ld resides on a read-only
/usr/obj mount.



--------------------------------------------------------------
>>> Installing everything
--------------------------------------------------------------
cd /usr/src; /usr/obj/usr/src/make.amd64/bmake -f Makefile.inc1 install
===> lib (install)
===> lib/csu (install)
===> lib/csu/amd64 (install)
install -o root -g wheel  -m 444 crt1.o crti.o crtn.o Scrt1.o gcrt1.o
/usr/lib/
===> lib/libc (install)
install -C -o root -g wheel -m 444   libc.a /usr/lib/
install -s -o root -g wheel -m 444   -fschg -S  libc.so.7 /lib/
install -T debug -o root -g wheel -m 444    libc.so.7.debug
/usr/lib/debug/lib/
install -S -C -o root -g wheel -m 444   libc.ld  /usr/lib/libc.so
rm -f libc.ld
rm: libc.ld: Permission denied
*** Error code 1

Stop.
bmake[5]: stopped in /usr/src/lib/libc
*** Error code 1

Stop.
bmake[4]: stopped in /usr/src/lib
*** Error code 1

Stop.
bmake[3]: stopped in /usr/src
*** Error code 1

Stop.
bmake[2]: stopped in /usr/src
*** Error code 1

Stop.
bmake[1]: stopped in /usr/src
*** Error code 1

Stop.
make: stopped in /usr/src
0.778u 0.483s 0:01.36 91.9%     1356+245k 0+655io 1483pf+0w
jsli_at_jsli-bsd:/usr/src #

-Jia-Shiun