gdb/arm: Add support for multiple stack pointers on Cortex-M

gdb/arm: Add support for multiple stack pointers on Cortex-M

Armv8-M architecture with Security extension features four stack pointers
to handle Secure and Non-secure modes.

This patch adds support to switch between them as needed during
unwinding, and replaces all updates of cache->prev_sp with calls to
arm_cache_set_prev_sp.

Signed-off-by: Torbjörn Svensson <torbjorn.svensson@st.com>
Signed-off-by: Christophe Lyon <christophe.lyon@foss.st.com>
Signed-off-by: Christophe Lyon <christophe.lyon@arm.com>