gdb/arm: Only stack S16..S31 when FPU registers are secure

The FPCCR.TS bit is used to identify if FPU registers are considered
non-secure or secure.  If they are secure, then callee saved registers
(S16 to S31) are stacked on exception entry or otherwise skipped.

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