aarch64: optimized memcpy implementation for thunderx2

Since aligned loads and stores are huge performance
advantage the implementation always tries to do aligned
access. Among the cases when src and dst addresses are
aligned or unaligned evenly there are cases of not evenly
unaligned src and dst. For such cases (if the length is
big enough) ext instruction is used to merge-and-shift
two memory chunks loaded from two adjacent aligned
locations and then the adjusted chunk gets stored to
aligned address.

Performance gain against the current T2 implementation:
     memcpy-large: 65K-32M: +40% - +10%
     memcpy-walk:  128-32M: +20% - +2%