Adjust struct member offset for i386 Cygwin

Fixes [Bug #17606]

vm_core.h

@@ -218,6 +218,10 @@ struct rb_control_frame_struct;
 /* iseq data type */
 typedef struct rb_compile_option_struct rb_compile_option_t;
 
+#if (SIZEOF_SERIAL_T > SIZEOF_VOIDP) && defined(__CYGWIN__)
+#pragma pack(push, 4) /* == SIZEOF_VOIDP */
+#endif
+
 // imemo_constcache
 struct iseq_inline_constant_cache_entry {
     VALUE flags;
@@ -228,6 +232,10 @@ struct iseq_inline_constant_cache_entry {
                               // v3
 };
 
+#if (SIZEOF_SERIAL_T > SIZEOF_VOIDP) && defined(__CYGWIN__)
+#pragma pack(pop)
+#endif
+
 struct iseq_inline_constant_cache {
     struct iseq_inline_constant_cache_entry *entry;
 };