test/ruby/test_jit_debug.rb: Skip unsupported platforms

Author: mame

test/ruby/test_jit_debug.rb

@@ -8,6 +8,9 @@
 
 class TestJITDebug < TestJIT
   def setup
+    unless JITSupport.supported?
+      skip 'JIT seems not supported on this platform'
+    end
     # let `#eval_with_jit` use --jit-debug
     @jit_debug = true
   end