Compile iseq_collector.c logic only on CRuby

* iseq_collector.c relies on many CRuby internals, there is no point to compile it on other Rubies.
* It now fails to compile on TruffleRuby:
  ../../../../ext/debug/iseq_collector.c: In function ‘imemo_type’:
  ../../../../ext/debug/iseq_collector.c:18:13: error: implicit declaration of function ‘RBASIC’ [-Werror=implicit-function-declaration]
    18 |     return (RBASIC(imemo)->flags >> FL_USHIFT) & imemo_mask;
        |            ^~~~~~

ext/debug/debug.c

@@ -180,7 +180,9 @@ iseq_last_line(VALUE iseqw)
 }
 #endif
 
+#ifdef CRUBY
 void Init_iseq_collector(void);
+#endif
 
 void
 Init_debug(void)
@@ -210,5 +212,7 @@ Init_debug(void)
     rb_define_method(rb_cISeq, "last_line", iseq_last_line, 0);
 #endif
 
+#ifdef CRUBY
     Init_iseq_collector();
+#endif
 }

ext/debug/extconf.rb

@@ -4,6 +4,7 @@
 $distcleanfiles << "debug_version.h"
 
 if defined? RubyVM
+  $defs << '-DCRUBY'
   $defs << '-DHAVE_RB_ISEQ_PARAMETERS'
   $defs << '-DHAVE_RB_ISEQ_CODE_LOCATION'
 

ext/debug/iseq_collector.c

@@ -1,3 +1,4 @@
+#ifdef CRUBY
 #include <ruby/ruby.h>
 
 VALUE rb_iseqw_new(VALUE v);
@@ -89,3 +90,4 @@ Init_iseq_collector(void)
     rb_define_singleton_method(rb_mObjSpace, "each_iseq", each_iseq, 0);
     rb_define_singleton_method(rb_mObjSpace, "count_iseq", count_iseq, 0);
 }
+#endif