devel/tigcc: Unbreak build with GCC 10 (-fno-common)

/usr/local/bin/ld: libbackend.a(m68k.o):(.bss+0x50): multiple definition of `m68k_regparm_string'; libbackend.a(options.o):(.bss+0x10): first defined here

http://package18.nyi.freebsd.org/data/121amd64-default-PR244494/2020-06-10_18h04m44s/logs/errors/tigcc-0.96.b8_10.log