gdb/arm: Simplify logic for updating addresses

Small performance improvement by fetching previous SP value only
once before the loop and reuse it to avoid fetching at every
iteration.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@st.com>
Signed-off-by: Yvan Roux <yvan.roux@foss.st.com>