[ping] Change static chain to r11 on aarch64

[Olivier Hainque]

Hi Wilco,

(and thanks everyone for the interesting input on this)

> On 17 Dec 2018, at 14:55, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
> 
> The AArch64 ABI defines x18 as platform specific:
[...]
> Using x9 would make its use as an extra argument clearer.

I'm experimenting with the idea of adjusting the
stack probing code using r9 today, to see if it could
save/restore that reg if it happens to be the static chain
as well.

If that can be made to work, maybe that would be a better
alternative than just swapping and have the stack probing
code use r10 and r11 instead (1 fewer register with dedicated
use).

Olivier