gdb/arm: Update active msp/psp when switching stack

For targets with secext, msp and psp can be seen as an alias for one
of msp_s, msp_ns, psp_s or psp_ns. When switching active sp, the
corresponding msp/psp needs to be switched too.

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