py, unix: Allow to compile with -Wsign-compare.

See issue #699.
diff --git a/py/asmx86.c b/py/asmx86.c
index d35712e..755ed03 100644
--- a/py/asmx86.c
+++ b/py/asmx86.c
@@ -398,7 +398,7 @@
     assert(label < as->max_num_labels);
     if (as->pass < ASM_X86_PASS_EMIT) {
         // assign label offset
-        assert(as->label_offsets[label] == -1);
+        assert(as->label_offsets[label] == (mp_uint_t)-1);
         as->label_offsets[label] = as->code_offset;
     } else {
         // ensure label offset has not changed from PASS_COMPUTE to PASS_EMIT
@@ -407,7 +407,7 @@
     }
 }
 
-STATIC mp_uint_t get_label_dest(asm_x86_t *as, int label) {
+STATIC mp_uint_t get_label_dest(asm_x86_t *as, mp_uint_t label) {
     assert(label < as->max_num_labels);
     return as->label_offsets[label];
 }
@@ -415,7 +415,7 @@
 void asm_x86_jmp_label(asm_x86_t *as, mp_uint_t label) {
     mp_uint_t dest = get_label_dest(as, label);
     mp_int_t rel = dest - as->code_offset;
-    if (dest != -1 && rel < 0) {
+    if (dest != (mp_uint_t)-1 && rel < 0) {
         // is a backwards jump, so we know the size of the jump on the first pass
         // calculate rel assuming 8 bit relative jump
         rel -= 2;
@@ -437,7 +437,7 @@
 void asm_x86_jcc_label(asm_x86_t *as, mp_uint_t jcc_type, mp_uint_t label) {
     mp_uint_t dest = get_label_dest(as, label);
     mp_int_t rel = dest - as->code_offset;
-    if (dest != -1 && rel < 0) {
+    if (dest != (mp_uint_t)-1 && rel < 0) {
         // is a backwards jump, so we know the size of the jump on the first pass
         // calculate rel assuming 8 bit relative jump
         rel -= 2;