PXOR doesn't work on YMM registers

[Marco Heisig]

This bug is triggered by any code that contains a (simd-pack-256 integer) compile-time constant of all zeros, e.g.,

(defun foo ()
  (let ((c (sb-ext:%make-simd-pack-256-ub64 0 0 0 0)))
    (multiple-value-list
     (sb-ext:%simd-pack-256-ub64s c))))

The attached patch replaces the invalid two argument SSE instruction pxor with the three argument AVX instruction vpxor.