Clarify these are just for MJIT

and not for third-party libraries.

See: e6484a1530

vm_core.h

@@ -1764,7 +1764,7 @@ rb_execution_context_t *rb_vm_main_ractor_ec(rb_vm_t *vm); // ractor.c
 /* for thread */
 
 #if RUBY_VM_THREAD_MODEL == 2
-RUBY_SYMBOL_EXPORT_BEGIN
+MJIT_SYMBOL_EXPORT_BEGIN
 
 RUBY_EXTERN struct rb_ractor_struct *ruby_single_main_ractor; // ractor.c
 RUBY_EXTERN rb_vm_t *ruby_current_vm_ptr;
@@ -1772,7 +1772,7 @@ RUBY_EXTERN rb_event_flag_t ruby_vm_event_flags;
 RUBY_EXTERN rb_event_flag_t ruby_vm_event_enabled_global_flags;
 RUBY_EXTERN unsigned int    ruby_vm_event_local_num;
 
-RUBY_SYMBOL_EXPORT_END
+MJIT_SYMBOL_EXPORT_END
 
 #define GET_VM()     rb_current_vm()
 #define GET_RACTOR() rb_current_ractor()

vm_insnhelper.h

@@ -11,13 +11,13 @@
 
 **********************************************************************/
 
-RUBY_SYMBOL_EXPORT_BEGIN
+MJIT_SYMBOL_EXPORT_BEGIN
 
 RUBY_EXTERN VALUE ruby_vm_const_missing_count;
 RUBY_EXTERN rb_serial_t ruby_vm_global_constant_state;
 RUBY_EXTERN rb_serial_t ruby_vm_class_serial;
 
-RUBY_SYMBOL_EXPORT_END
+MJIT_SYMBOL_EXPORT_END
 
 #if VM_COLLECT_USAGE_DETAILS
 #define COLLECT_USAGE_INSN(insn)           vm_collect_usage_insn(insn)