botan: disable AltiVec if the CPU does not support it

botan: disable AltiVec if the CPU does not support it

Botan compiles all PowerPC code with AltiVec support and only determines
at runtime whether the CPU even supports it. If the target CPU uses the
older SPE extensions, though, this is too late; since SPE and AltiVec
are incompatible, gcc aborts the compilation. This patch explicitly
disables AltiVec support unless BR2_POWERPC_CPU_HAS_ALTIVEC is defined

Signed-off-by: Tobias Blass <tobias.blass@rohde-schwarz.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>