target/ppc: simplify VEXT_SIGNED macro in int_helper.c
As pointed out by Richard: it does not need the mask argument, nor does it need
the recast argument. The masking is implied by the cast argument, and the
recast is implied by the assignment.
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>