[C++] Add casts to obstack_base calls

The recent libiberty import of upstream obstack.h (314dee8ea9) makes
obstack_base return a 'void *', with the consequence that a few places
in gdb need a (char *) cast.

gdb/ChangeLog:
2015-11-18  Simon Marchi  <simon.marchi@ericsson.com>
	    Pedro Alves  <palves@redhat.com>

	* break-catch-sig.c (signal_catchpoint_print_one): Add cast.
	* c-exp.y (parse_string_or_char, yylex): Add casts.
	* c-lang.c (evaluate_subexp_c): Add casts.
	* d-exp.y (parse_string_or_char, yylex): Add casts.
	* go-exp.y (parse_string_or_char, build_packaged_name): Add casts.
	* p-valprint.c (pascal_object_print_value_fields): Add casts.
	* valprint.c (generic_emit_char, generic_printstr): Add casts.

gdb/c-lang.c

@@ -701,7 +701,7 @@ evaluate_subexp_c (struct type *expect_type, struct expression *exp,
 			obstack_object_size (&output));
 	      }
 	    else
-	      result = value_cstring (obstack_base (&output),
+	      result = value_cstring ((const char *) obstack_base (&output),
 				      obstack_object_size (&output),
 				      type);
 	  }