gdb/arm: Unwind Non-Secure callbacks from Secure

gdb/arm: Unwind Non-Secure callbacks from Secure

Without this changeset, the unwinding doesn't take into account
Non-Secure to Secure stack unwinding enablement status and
doesn't choose the proper SP to do the unwinding.

This patch only unwinds the stack when Non-Secure to Secure
unwinding is enabled, previous SP is set w/r to the current mode
(Handler -> msp_s, Thread -> psp_s) and then the Secure stack is
unwound.  Ensure thumb bit is set in PSR when needed.  Also, drop
thumb bit from PC if set.

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