[Ada] Gigi support for OpenACC pragmas

[Olivier Hainque]

(A message that remained in my drafts box)

> On 3 Dec 2018, at 16:50, Pierre-Marie de Rodat <derodat@adacore.com> wrote:
> 
> Matching front-end bits to support Acc_Kernels, Acc_Parallel,
> Acc_Loop and Acc_Data.

Note that this was all originally contributed by Ghujan Lacambre
(<ghjuvan.lacambre@etu.upmc.fr>)

Olivier