py/stackctrl: Add mp_stack_set_top() to explicitly set stack top value.

Useful for embedded targets with fixed stack layout.

py/stackctrl.c

@@ -35,6 +35,10 @@ void mp_stack_ctrl_init(void) {
     MP_STATE_VM(stack_top) = (char*)&stack_dummy;
 }
 
+void mp_stack_set_top(void *top) {
+    MP_STATE_VM(stack_top) = top;
+}
+
 mp_uint_t mp_stack_usage(void) {
     // Assumes descending stack
     volatile int stack_dummy;

py/stackctrl.h

@@ -29,6 +29,7 @@
 #include "py/mpconfig.h"
 
 void mp_stack_ctrl_init(void);
+void mp_stack_set_top(void *top);
 mp_uint_t mp_stack_usage(void);
 
 #if MICROPY_STACK_CHECK