Fix regression from commit 69332fb (#1123)

Commit 69332fb is a cherry-pick of bellard/quickjs@9bd10d8 but changed
the type of a field from uint8_t to bool. That normally would have been
fine except that the layouts of JSVarRef and JSGCObjectHeader need to
match. They did on architectures where sizeof(bool) == sizeof(char) but
that is not the case on at least powerpc and probably others.

Author: barracuda156

quickjs.c

@@ -374,7 +374,7 @@ typedef struct JSVarRef {
         struct {
             int __gc_ref_count; /* corresponds to header.ref_count */
             uint8_t __gc_mark; /* corresponds to header.mark/gc_obj_type */
-            bool is_detached;
+            uint8_t is_detached;
         };
     };
     JSValue *pvalue; /* pointer to the value, either on the stack or