gas: bfin: build lexer with -Werror

gas: bfin: build lexer with -Werror

The makefile has comments about old versions of bison/yacc generating
warnings, but that doesn't apply to the lexer which comes from flex.
As far as I can tell, the warnings in the Blackfin lexer can be fixed
with defines that have been supported back through flex in 2002.  So
lets turn on -Werror for it and see if anyone notices.  If they do,
they can report their exact tool versions so we can record that here.