configure: only disable VSX for !ppc64el

This reverts commit 04f00022, which made it impossible to enable VSX with
a generic cpu.

This changes the behavior back to what it was before commit b0af4043.

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>