GCC 10 enables -fno-common by default. This resulted in a multiple
definition linker error since global variables were declared and defined
in a header file:
ld: ld-insn.o:sim/ppc/ld-insn.h:221: multiple definition of
`max_model_fields_len'; igen.o:sim/ppc/ld-insn.h:221: first defined here