sh: get_user fixes and nommu consolidation.

When a get_user(to, from++) is called the pointer increment is performed
after its first usage, in the specific after the __add_ok invokation.
This causes a wrong get_user return value, putting a wrong character
in the destination variable. This patch solves the problem using a new
temporary pointer.

Additionally this reworks the use of the register banks, allowing for
consolidation between the MMU and nommu implementations.

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>
Signed-off-by: Giuseppe Condorelli <giuseppe.condorelli@st.com>
Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>