py/nlrthumb: Add support for iOS where the C func is _nlr_push_tail.

py/nlrthumb.c

@@ -72,7 +72,11 @@ __attribute__((naked)) unsigned int nlr_push(nlr_buf_t *nlr) {
     ".align 2                   \n"
     "nlr_push_tail_var: .word nlr_push_tail \n"
 #else
+    #if defined(__APPLE__) || defined(__MACH__)
+    "b      _nlr_push_tail      \n" // do the rest in C
+    #else
     "b      nlr_push_tail       \n" // do the rest in C
+    #endif
 #endif
     );