Hm, well, I was only half-right.   Consider the following,
even more peculiar, observations.

	Sean.

cc -pipe -O3 -march=i686 -mfancy-math-387 -fsched-interblock -fsched-spec -fsched-spec-load -fbranch-count-reg -fno-expensive-optimizations -fforce-mem -fforce-addr -finline-functions -mwide-multiply -funroll-loops -DALL_STATE  -Wall -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Wno-uninitialized -Werror   -D_LIBC -DNLS -DYP -DHESIOD -DLIBC_SCCS -DSYSLIBC_SCCS  -D_REENTRANT -I/usr/src/lib/libc/include -DINET6 -D__DBINTERFACE_PRIVATE -I/usr/src/lib/libc/../../libexec/ld.elf_so -I/usr/src/lib/libc/dlfcn -DRESOLVSORT -I. -DPOSIX_MISTAKE -DFLOATING_POINT -nostdinc -idirafter /usr/local/safeplace/usr/include -c -fPIC -DPIC /usr/src/lib/libc/yp/ypprot_err.c -o ypprot_err.so
sync; sleep 10; sync
building standard c library
sync; sleep 10; sync
/usr/bin/nm: getdents.o: File format not recognized
/usr/bin/nm: mknod.o: File truncated
/usr/bin/nm: __msgctl13.o: File format not recognized
/usr/bin/nm: msgrcv.o: File truncated
/usr/bin/nm: msgsnd.o: File truncated
/usr/bin/nm: __msync13.o: File format not recognized
/usr/bin/nm: munlock.o: File truncated
/usr/bin/nm: munlockall.o: File truncated
/usr/bin/nm: munmap.o: File truncated
/usr/bin/nm: profil.o: File truncated
/usr/bin/nm: setuid.o: File truncated
/usr/bin/nm: __shmctl13.o: File truncated
ar: libc.a: File truncated
sean# cd /usr/src/lib/libc
sean# make
sync; sleep 10; sync
building profiled c library
sync; sleep 10; sync
/usr/bin/nm: connect.po: File truncated
ar: libc_p.a: File truncated
*** Error code 1

Stop.
sean# make
sync; sleep 10; sync
building shared object c library
sync; sleep 10; sync
/usr/bin/nm: dup.so: File format not recognized
/usr/bin/nm: __msgctl13.so: File truncated
/usr/bin/nm: msgrcv.so: File truncated
/usr/bin/nm: msgsnd.so: File truncated
/usr/bin/nm: __msync13.so: File format not recognized
/usr/bin/nm: munlock.so: File truncated
/usr/bin/nm: munlockall.so: File truncated
/usr/bin/nm: munmap.so: File truncated
/usr/bin/nm: setuid.so: File truncated
/usr/bin/nm: __sigaltstack14.so: File truncated
/usr/bin/nm: write.so: File truncated
/usr/bin/nm: writev.so: File truncated
/usr/bin/nm: __posix_chown.so: File truncated
/usr/bin/nm: getegid.so: File truncated
/usr/bin/nm: issetugid.so: File truncated
/usr/bin/nm: bt_page.so: File format not recognized
/usr/bin/nm: db.so: File format not recognized
/usr/bin/nm: getdtablesize.so: File format not recognized
/usr/bin/nm: daemon.so: File format not recognized
/usr/bin/nm: execl.so: File truncated
/usr/bin/nm: execlp.so: File truncated
ar: libc_pic.a: File truncated
*** Error code 1

Stop.
sean# rm obj.i386/libc.a
sean# make
sync; sleep 10; sync
building standard c library
sync; sleep 10; sync
/usr/bin/nm: getdents.o: File format not recognized
/usr/bin/nm: mknod.o: File truncated
/usr/bin/nm: __msgctl13.o: File format not recognized
/usr/bin/nm: msgrcv.o: File truncated
/usr/bin/nm: msgsnd.o: File truncated
/usr/bin/nm: __msync13.o: File format not recognized
/usr/bin/nm: munlock.o: File truncated
/usr/bin/nm: munlockall.o: File truncated
/usr/bin/nm: munmap.o: File truncated
/usr/bin/nm: profil.o: File truncated
/usr/bin/nm: setuid.o: File truncated
/usr/bin/nm: __shmctl13.o: File truncated
ar: libc.a: File truncated
*** Error code 1
sean# pushd obj.i386
/usr/src/lib/libc/obj.i386 /usr/src/lib/libc 
sean# rm libc.a getdents.o mknod.o __msgctl13.o msgrcv.o msgsnd.o __msync13.o munlock.o munlockall.o munmap.o profil.o setuid.o __shmctl13.o
sean# popd
/usr/src/lib/libc 
sean# make
creating __msgctl13.o
creating __msync13.o
creating __shmctl13.o
creating getdents.o
creating mknod.o
creating msgrcv.o
creating msgsnd.o
creating munlock.o
creating munlockall.o
creating munmap.o
creating profil.o
creating setuid.o
sync; sleep 10; sync
building standard c library
sync; sleep 10; sync
/usr/bin/nm: wait4.o: File truncated
/usr/bin/nm: write.o: File truncated
/usr/bin/nm: writev.o: File truncated
/usr/bin/nm: __posix_chown.o: File format not recognized
/usr/bin/nm: __posix_lchown.o: File truncated
ar: libc.a: File truncated
*** Error code 1
sean# pushd obj.i386
/usr/src/lib/libc/obj.i386 /usr/src/lib/libc 
sean# rm wait4.o write.o writev.o __posix_chown.o __posix_lchown.o
sean# rm libc.a
sean# popd
/usr/src/lib/libc 
sean# make
creating __posix_chown.o
creating __posix_lchown.o
creating wait4.o
creating write.o
creating writev.o
sync; sleep 10; sync
building standard c library
sync; sleep 10; sync
/usr/bin/nm: __posix_rename.o: File truncated
ar: libc.a: File truncated
*** Error code 1