chore(wba): declare _sstack in linker script

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

variants/STM32WBAxx/WBA55C(E-G)U/ldscript.ld

@@ -40,6 +40,7 @@ _estack = ORIGIN(RAM) + LENGTH(RAM); /* end of "RAM" Ram type memory */
 
 _Min_Heap_Size = 0x200; /* required amount of heap */
 _Min_Stack_Size = 0x400; /* required amount of stack */
+_sstack = _estack - _Min_Stack_Size;
 
 /* Memories definition */
 MEMORY