Add altivec support (smx, IBM Power 8)

Targeting open power arch, we start porting the inastemp simd class to altivec.

It is currently a work in progress.


* [x]   Manage load/store (especially not alligned)
* [x]   Manage little/big Indian (in the load/store and the signof)

* [x]  Build with XL compiler

See merge request !3