py/asmbase: Revert removal of clearing of label offsets for native emit.

The assembler back-end for most architectures needs to know if a jump is
backwards in order to emit optimised machine code, and they do this by
checking if the destination label has been set or not.  So always reset
label offsets to -1 (this reverts partially the previous commit, with some
minor optimisation for the if-logic with the pass variable).

py/asmbase.c

@@ -46,12 +46,10 @@ void mp_asm_base_deinit(mp_asm_base_t *as, bool free_code) {
 }
 
 void mp_asm_base_start_pass(mp_asm_base_t *as, int pass) {
-    if (pass == MP_ASM_PASS_COMPUTE) {
-        #ifndef NDEBUG
-        // With debugging enabled labels are checked for unique assignment
+    if (pass < MP_ASM_PASS_EMIT) {
+        // Reset labels so we can detect backwards jumps (and verify unique assignment)
         memset(as->label_offsets, -1, as->max_num_labels * sizeof(size_t));
-        #endif
-    } else if (pass == MP_ASM_PASS_EMIT) {
+    } else {
         // allocating executable RAM is platform specific
         MP_PLAT_ALLOC_EXEC(as->code_offset, (void**)&as->code_base, &as->code_size);
         assert(as->code_base != NULL);