Bootstrapping ruby runtime might not have RubyVM::MJIT defined.

[Fix GH-1891]

From: U-DESKTOP-RLT5UQ8\moriyoshi <mozo@mozo.jp>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63664 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

template/fake.rb.in

@@ -32,7 +32,7 @@ class Object
   CROSS_COMPILING = RUBY_PLATFORM
   constants.grep(/^RUBY_/) {|n| remove_const n}
 % arg['versions'].each  {|n, v|
-  <%=n%> = <%if n=='RUBY_DESCRIPTION' %>RubyVM::MJIT.enabled? ?
+  <%=n%> = <%if n=='RUBY_DESCRIPTION' %>RubyVM.const_defined?(:MJIT) && RubyVM::MJIT.enabled? ?
     <%=arg['RUBY_DESCRIPTION_WITH_JIT'].inspect%> :
     <%end%><%=v.inspect%>
 % }