Use a union to avoid casts in bfd/doc/chew.c

Use a union to avoid casts in bfd/doc/chew.c

This fixes -Wpedantic warnings in chew.c.  Conversion between function
and object pointers is not guaranteed.  They can even be different
sizes, not that we're likely to encounter build machines like that
nowadays.

	PR 29194
	* doc/chew.c (pcu): New union typedef.
	(dict_type, pc): Use it here.  Adjust uses of pc.
	(add_to_definition): Make "word" param a pcu.  Adjust all uses
	of function.
	(stinst_type): Delete.