[9front] jpg build fails on rpi4

[sl@stanleylieber.com]

for (i in jpg tojpg gif togif ppm toppm png topng tif totif yuv ico toico bmp tga v210)
	mk $MKFLAGS $i.install
7^c rgbycc.c
7^l -o 7.rgbycc rgbycc.7
./7.rgbycc > ycbcr.h
7^c rgbrgbv.c
7^l -o 7.rgbrgbv rgbrgbv.7
./7.rgbrgbv > rgbv.h
7c -FTVw totruecolor.c
7c -FTVw writerawimage.c
7c -FTVw readjpg.c
7c -FTVw jpg.c
7c -FTVw torgbv.c
7l  -o 7.jpg torgbv.7 totruecolor.7 writerawimage.7 readjpg.7 jpg.7
cp 7.jpg /arm64/bin/jpg
7c -FTVw writejpg.c
7c -FTVw multichan.c
7c -FTVw tojpg.c
7l  -o 7.tojpg writejpg.7 multichan.7 tojpg.7
cp 7.tojpg /arm64/bin/tojpg
7c -FTVw torgbv.c
7c -FTVw readgif.c
7c -FTVw gif.c
7l  -o 7.gif torgbv.7 totruecolor.7 writerawimage.7 readgif.7 gif.7
cp 7.gif /arm64/bin/gif
7c -FTVw writegif.c
7c -FTVw onechan.c
7c -FTVw togif.c
7l  -o 7.togif writegif.7 onechan.7 togif.7 torgbv.7
cp 7.togif /arm64/bin/togif
7c -FTVw readppm.c
7c -FTVw ppm.c
7l  -o 7.ppm torgbv.7 totruecolor.7 writerawimage.7 readppm.7 ppm.7
unknown type -14
readpixmap: truncated object file: readppm.7
readpixmap: truncated object file: ppm.7
_main: undefined: main
(18)	BL	,main+0(SB)
getpid: initialize non-data (5): .string
(1676)	DATA	.string<>+0(SB)/8,$"\zout"
getpid: initialize bounds (0): .string
(1676)	DATA	.string<>+0(SB)/8,$"\zout"
Bgetbit: undefined external: nbit in Bgetbit
Bgetbit: undefined external: bitc in Bgetbit
readpixmap: undefined external: .string in readpixmap