Manuel Bouyer found the solution to this problem, so I thought I 
would report it here so it appears in the archives:

At 12:09 PM -0700 5/10/01, Paul Hoffman wrote:
>Greetings again. After some wonderful help from Manuel Bouyer on the 
>problem I reported last night, I need to make some installation 
>floppies to test whether it fixes my problem. Unfortunately, the 
>floppy-making script doesn't finish. In 
>/usr/src/distrib/i386/floppies, I say "make", and it makes fine 
>until:
>
>(cd 
>/usr/src/distrib/i386/floppies/ramdisk-big/../../../../sys/arch/i386/stand/installboot; 
>make installboot.o bootblks.o getmount.o loadfile.o)
>cc -g -Wall -Wmissing-prototypes -Wstrict-prototypes  -Werror 
>-I/usr/src/sys/arch/i386/stand/installboot/../lib/crt/bootsect 
>-I/usr/src/sys/arch/i386/stand/installboot/../../../../lib/libsa -I. 
>-c installboot.c
>In file included from installboot.c:59:
>/usr/src/sys/arch/i386/stand/installboot/../../../../lib/libsa/loadfile.h:70: 
>machine/loadfile_machdep.h: No such file or directory
>*** Error code 1
>
>Stop.
>*** Error code 1
>
>Stop.
>*** Error code 1
>
>Stop.
>*** Error code 1
>
>Stop.
>
>How can I move forwards from here?

Answer: do a 'make' in sys/arch/i386/stand.