gdb/arm: Fetch initial sp value prior to compare

gdb/arm: Fetch initial sp value prior to compare

For Arm Cortex-M33 with security extensions, there are 4 different
stack pointers (msp_s, msp_ns, psp_s, psp_ns).  In order to
identify the active one, compare the values of the different
stacks. The value of the initial sp register needs to be fetched to
perform this comparison.

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