[SystemZ] Fix vecintrin.h to not emit alignment hints in vec_xl/vec_xst.

vec_xl() and vec_xst() should not emit alignment hints since they take a
scalar pointer and also add a byte offset if passed.

This patch uses memcpy to achieve the desired result.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D96471

Author: JonPsson