sh: fix endianess and optimise the SH4 memcpy

sh: fix endianess and optimise the SH4 memcpy

This patch fixes the big-endian code and adds a new optimization
only for little endian mode.
This optimization is based on prefetching and 64bit data transfer via FPU.
Tests shows that

----------------------------------------
 Memory bandwidth    |        Gain
                     | sh4-300 | sh4-200
----------------------------------------
 512 bytes to 16KiB  | ~20%    | ~25%
 from 32KiB to 16MiB | ~190%   | ~5%
----------------------------------------

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>