Export RbConfig::CONFIG["COROUTINE_TYPE"]

THREAD_MODEL is exported already, so this matches that.  Exporting this
is simpler than inspecting configure_args and arch and matching that up
with a specific configure.ac.

Fix GH-5976

configure.ac

@@ -4007,6 +4007,7 @@ AC_SUBST(EXPORT_PREFIX)
 AC_SUBST(SYMBOL_PREFIX)
 AC_SUBST(MINIOBJS)
 AC_SUBST(THREAD_MODEL)
+AC_SUBST(COROUTINE_TYPE, ${coroutine_type})
 AC_SUBST(PLATFORM_DIR)
 
 firstmf=`echo $FIRSTMAKEFILE | sed 's/:.*//'`