[PATCH][AArch64] Document +rcpc weak release consistency extension

[Nick Clifton]

Hi Szabolcs,

> gas/
> 2017-03-01  Szabolcs Nagy  <szabolcs.nagy@arm.com>
> 
> 	* doc/c-aarch64.texi (AArch64 Extensions): Document rcpc.

Approved - please apply.  (I would consider this patch to be "obvious").

Cheers
  Nick