gdb/arm: Make sp alias for one of the other stack pointers

For Cortex-M targets, SP register is never detached from msp or
psp, it always has the same value as one of them.  Let GDB treat
ARM_SP_REGNUM as an alias similar to what is done in hardware.

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