py: Add new cstack API for stack checking, with limit margin macro.

Currently the stack limit margin is hard-coded in each port's call to
`mp_stack_set_limit()`, but on threaded ports it's fiddlier and can lead to
bugs (such as incorrect thread stack margin on esp32).

This commit provides a new API to initialise the C Stack in one function
call, with a config macro to set the margin.  Where possible the new call
is inlined to reduce code size in thread-free ports.

Intended replacement for `MP_TASK_STACK_LIMIT_MARGIN` on esp32.

The previous `stackctrl.h` API is still present and unmodified apart from a
deprecation comment.  However it's not available when the
`MICROPY_PREVIEW_VERSION_2` macro is set.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

examples/natmod/re/re.c

@@ -11,7 +11,7 @@
 
 const char *stack_top;
 
-void mp_stack_check(void) {
+void mp_cstack_check(void) {
     // Assumes descending stack on target
     volatile char dummy;
     if (stack_top - &dummy >= STACK_LIMIT) {