lib/raid6: Fix arm64 test build

The lib/raid6/test fails to build the neon objects
on arm64 because the correct machine type is 'aarch64'.

Once this is correctly enabled, the neon recovery objects
need to be added to the build.

Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>