Fix unwinding for Thumb-2 libraries

[Richard Henderson]

>     We can't use push/pop inside the asm because that breaks
> +   unwinding (i.e. thread cancellation) for this frame.  We can't
> +   locally save and restore r7, because we do not know if this
> +   function uses r7 or if it is our caller's r7; if it is our caller's,
> +   then unwinding will fail higher up the stack.  So we move the
> +   syscall out of line and provide its own unwind information.  */

If you've got cfi directives, you *can* manipulate the unwind
info inside the asm within the current frame.


r~