gas: improve interaction between read_a_source_file() and s_linefile()

read_a_source_file() would bump line numbers only when seeing a newline,
whereas is_end_of_line[] indicates further end-of-line characters, in
particular the nul character. s_linefile() attempts to compensate for
the bump, but was too aggressive with this so far: It should only adjust
when a newline ends the line. To facilitate such a check, the check for
nothing else on the line needs to move ahead, which luckily is easily
possible: The relevant two conditions match, and the function can
simply return from the body of that earlier instance of the conditional.

The more strict treatment in s_linefile() then requires an adjustment
to buffer_and_nest()'s invocation of the function: The line terminator
now needs to be a newline, not nul.

gas/macro.c

@@ -265,7 +265,7 @@ buffer_and_nest (const char *from, const char *to, sb *ptr,
 	    {
 	      char saved_eol_char = ptr->ptr[ptr->len];
 
-	      ptr->ptr[ptr->len] = '\0';
+	      ptr->ptr[ptr->len] = '\n';
 	      temp_ilp (ptr->ptr + i + 8);
 	      s_linefile (0);
 	      restore_ilp ();

gas/read.c

@@ -2047,18 +2047,22 @@ s_linefile (int ignore ATTRIBUTE_UNUSED)
 
       if (file || flags)
 	{
-	  linenum--;
+	  demand_empty_rest_of_line ();
+
+	  /* read_a_source_file() will bump the line number only if the line
+	     is terminated by '\n'.  */
+	  if (input_line_pointer[-1] == '\n')
+	    linenum--;
+
 	  new_logical_line_flags (file, linenum, flags);
 #ifdef LISTING
 	  if (listing)
 	    listing_source_line (linenum);
 #endif
+	  return;
 	}
     }
-  if (file || flags)
-    demand_empty_rest_of_line ();
-  else
-    ignore_rest_of_line ();
+  ignore_rest_of_line ();
 }
 
 /* Handle the .end pseudo-op.  Actually, the real work is done in