[split] Support linker patching

[Jay Foad]

> +/* Emit a special section directive to indicate that this object file
> +   was compiled with -fsplit-stack.  This is used to let the linker
> +   detect calls between split-stack code and non-split-stack code, so
> +   that it can modify the split-stack code to allocate a sufficiently
> +   large stack.  We make the section executable if there are any
> +   functions in this file which have the no_split_stack attribute, to
> +   prevent the linker from warning about being unable to convert the
> +   functions if they call non-split-stack code.  */

Is the "we make the section executable" bit out of date?

Jay.