tcg/ppc: Enable Altivec detection

Now that we have implemented the required tcg operations,
we can enable detection of host vector support.

Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> (PPC32)
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>