I'm not sure if this is a bug really, but rather strange that GNU binutils cannot be build with GBU compiler.
Well, I've tried gcc7, gcc8, gcc10. Only very old gcc-4.8 can conpile it without the error:

/bin/sh ./libtool  --tag=CC   --mode=link gcc10 -W -Wall -Wstrict-prototypes -Wmissing-prototypes -Wshadow -Wstack-usage=262144  -O2 -pipe -march=bdver2  -fstack-protector-strong -Wl,-rpath=/usr/local/lib/gcc10 -isystem /usr/local/include -fno-strict-aliasing   -fstack-protector-strong -Wl,-rpath=/usr/local/lib/gcc10 -L/usr/local/lib/gcc10  -o coffdump coffdump.o coffgrok.o bucomm.o version.o filemode.o ../bfd/libbfd.la ../libiberty/libiberty.a  -L/usr/local/lib
libtool: link: gcc10 -W -Wall -Wstrict-prototypes -Wmissing-prototypes -Wshadow -Wstack-usage=262144 -O2 -pipe -march=bdver2 -fstack-protector-strong -Wl,-rpath=/usr/local/lib/gcc10 -isystem /usr/local/include -fno-strict-aliasing -fstack-protector-strong -Wl,-rpath=/usr/local/lib/gcc10 -o coffdump coffdump.o coffgrok.o bucomm.o version.o filemode.o  -L/usr/local/lib/gcc10 ../bfd/.libs/libbfd.a -L/usr/local/lib -lz ../libiberty/libiberty.a
/usr/local/bin/ld: bucomm.o:(.bss+0x0): multiple definition of `program_name'; coffdump.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status
gmake[6]: *** [Makefile:891: coffdump] Error 1
gmake[6]: Leaving directory '/usr/ports/devel/binutils/work-native/binutils-2.33.1/binutils'