gdb: add type::has_varargs / type::set_has_varargs

gdb: add type::has_varargs / type::set_has_varargs

Add the `has_varargs` and `set_has_varargs` methods on `struct type`, in
order to remove the `TYPE_VARARGS` macro.  In this patch, the macro is
changed to use the getter, so all the call sites of the macro that are
used as a setter are changed to use the setter method directly.  The
next patch will remove the macro completely.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <has_varargs, set_has_varargs>: New methods.
	(TYPE_VARARGS): Use type::has_varargs, change all write call sites to
	use type::set_has_varargs.

Change-Id: I898a1093ae40808b37a7c6fced7f6fa2aae604de