IA-16: add assume_ds_data', no_assume_ds_data' func. attribs.

This allows one to specify whether each individual function
assumes %ds == .data.

Hopefully this change will make it easier, in the future, to
add support for expressing far callbacks, far IRQ handlers,
etc. (#19).

The test cases and documentation have also been cleaned up
and updated.

gcc/config/ia16/ia16-c.c

@@ -76,6 +76,8 @@ ia16_cpu_cpp_builtins (void)
   cpp_define (parse_in, "__IA16_FEATURE_FAR_FUNCTION_SYNTAX");
   cpp_define (parse_in, "__IA16_FEATURE_ATTRIBUTE_CDECL");
   cpp_define (parse_in, "__IA16_FEATURE_ATTRIBUTE_STDCALL");
+  cpp_define (parse_in, "__IA16_FEATURE_ATTRIBUTE_ASSUME_DS_DATA");
+  cpp_define (parse_in, "__IA16_FEATURE_ATTRIBUTE_NO_ASSUME_DS_DATA");
 
   /* Also define a macro to give the function calling convention settings
      in use.  */

gcc/config/ia16/ia16-protos.h

@@ -24,9 +24,7 @@ extern struct gcc_target targetm;
 extern unsigned char	ia16_hard_regno_nregs[17][FIRST_PSEUDO_REGISTER];
 extern enum reg_class const ia16_regno_class[FIRST_PSEUDO_REGISTER];
 extern int		ia16_regno_in_class_p (unsigned, unsigned);
-extern int		ia16_far_function_type_p (const_tree funtype);
 extern int		ia16_in_far_function_p (void);
-extern int		ia16_far_function_rtx_p (rtx addr);
 extern bool		ia16_have_seg_override_p (rtx x);
 extern HOST_WIDE_INT	ia16_first_parm_offset (tree fundecl);
 extern HOST_WIDE_INT	ia16_initial_frame_pointer_offset (void);
@@ -62,7 +60,7 @@ extern bool	ia16_non_overlapping_mem_p (rtx m1, rtx m2);
 #endif
 
 extern void	ia16_expand_prologue (void);
-extern void	ia16_expand_reset_ds_for_call (void);
+extern void	ia16_expand_reset_ds_for_call (rtx addr);
 extern void	ia16_expand_epilogue (bool sibcall);
 extern const char *
 		ia16_get_call_expansion (rtx, machine_mode, bool);