Get rid of redefinition of `rb_execution_context_t`

Regardless of the order to include "vm_core.h" and "builtin.h".

builtin.h

@@ -22,8 +22,9 @@ struct rb_builtin_function {
 
 void rb_load_with_builtin_functions(const char *feature_name, const struct rb_builtin_function *table);
 
-#ifndef VM_CORE_H_EC_DEFINED
+#ifndef rb_execution_context_t
 typedef struct rb_execution_context_struct rb_execution_context_t;
+#define rb_execution_context_t rb_execution_context_t
 #endif
 
 /* The following code is generated by the following Ruby script:

vm_core.h

@@ -827,7 +827,7 @@ typedef char rb_thread_id_string_t[sizeof(rb_nativethread_id_t) * 2 + 3];
 
 typedef struct rb_fiber_struct rb_fiber_t;
 
-typedef struct rb_execution_context_struct {
+struct rb_execution_context_struct {
     /* execution information */
     VALUE *vm_stack;		/* must free, must mark */
     size_t vm_stack_size;       /* size in word (byte size / sizeof(VALUE)) */
@@ -876,7 +876,12 @@ typedef struct rb_execution_context_struct {
 	size_t stack_maxsize;
 	RUBY_ALIGNAS(SIZEOF_VALUE) jmp_buf regs;
     } machine;
-} rb_execution_context_t;
+};
+
+#ifndef rb_execution_context_t
+typedef struct rb_execution_context_struct rb_execution_context_t;
+#define rb_execution_context_t rb_execution_context_t
+#endif
 
 // for builtin.h
 #define VM_CORE_H_EC_DEFINED 1