Add a exception handling helper code

This commit add a structure definition that represents the stack frame
layout as it is upon entering exception handler. This is useful when
one wants to alter the exception return address, or get information
about registers state saved by the CPU.

include/libopencm3/cm3/scb.h

@@ -365,6 +365,23 @@
 
 BEGIN_DECLS
 
+struct scb_exception_stack_frame {
+	u32 r0;
+	u32 r1;
+	u32 r2;
+	u32 r3;
+	u32 r12;
+	u32 lr;
+	u32 pc;
+	u32 xpsr;
+} __attribute__((packed));
+
+#define SCB_GET_EXCEPTION_STACK_FRAME(f)				\
+	do {								\
+		asm volatile ("mov %[frameptr], sp"			\
+			      : [frameptr]"=r" (f));			\
+	} while (0)
+
 void scb_reset_core(void);
 void scb_reset_system(void);
 void scb_set_priority_grouping(u32 prigroup);