selftests/powerpc: Build the copyloops with -maltivec

The recent change to remove the vrX defines exposed the fact that we are
building the copyloops tests without altivec enabled. It depends on the
toolchain as to whether altivec is on by default or not, so it only
breaks on some toolchains. But we should always enable it.

Fixes: c2ce6f9f ("powerpc: Change vrX register defines to vX to match gcc and glibc")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>