package/aircrack-ng: powerpc support optional ALTIVEC

This patch adds support for the powerpc arch to conditionally
check if an arch provies altivec accelerator support, similar
to other SIMD on ARM/x86.

Upstream issue: aircrack-ng/aircrack-ng#1941

Fixes
http://autobuild.buildroot.net/results/87e82a5e8d0b1c1ff10ec3e59d25bcd56b329075



Tested against both a e6500 with Altivec and a e500 target.

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>