gdb/arm: Sync sp with other *sp registers

gdb/arm: Sync sp with other *sp registers

For Arm Cortex-M33 with security extensions, there are 4 different
stack pointers (msp_s, msp_ns, psp_s, psp_ns), without security
extensions and for other Cortex-M targets, there are 2 different
stack pointers (msp and psp).

With this patch, sp will always be in sync with one of the real stack
pointers on Arm targets that contain more than one stack pointer.

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