gdb/arm: Update the value of active sp when base sp changes

gdb/arm: Update the value of active sp when base sp changes

For Arm Cortex-M33 with security extensions, there are 4 different
stacks pointers (msp_s, msp_ns, psp_s, psp_ns).
When plain "sp" is updated during unwinding of the stack, the active
stack pointer of the 4 stack pointers needs to be kept in sync.

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