nlr: Add implementation using setjmp/longjmp.

Having an optimized asm implementation is good, but if we want portability,
that's it.

py/nlrsetjmp.c

@@ -0,0 +1,16 @@
+#include <setjmp.h>
+#include <stdio.h>
+#include "nlr.h"
+
+#if MICROPY_NLR_SETJMP
+
+nlr_buf_t *nlr_setjmp_top;
+
+void nlr_setjmp_jump(void *val) {
+    nlr_buf_t *buf = nlr_setjmp_top;
+    nlr_setjmp_top = buf->prev;
+    buf->ret_val = val;
+    longjmp(buf->jmpbuf, 1);
+}
+
+#endif