configure.ac: Detect if running on POWER8 arch

configure.ac: Detect if running on POWER8 arch

To determine if we could use special POWER8 assembly directives, we first
need to detect whether we are running on POWER8 architecture. This patch
adds this detection to configure.ac and adds the necessary compilation
flags accordingly.

v2:

- Add option to disable POWER8 instructions generation
- Detect whether building on BE or LE machine and build with
  -mpower8-vector only on LE machine
- Make the printed messages more standard

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>