fortran: New predicate gfc_length_one_character_type_p

Introduce a new predicate to simplify conditionals checking for
a character type whose length is the constant one.

gcc/fortran/ChangeLog:

	* gfortran.h (gfc_length_one_character_type_p): New inline
	function.
	* check.cc (is_c_interoperable): Use
	gfc_length_one_character_type_p.
	* decl.cc (verify_bind_c_sym): Same.
	* trans-expr.cc (gfc_conv_procedure_call): Same.