* YJIT: print msg to stderr when RubyVM::YJIT.disasm not available

Print a more useful error message when people try to use this
feature without YJIT dev.

Also fix an issue with .gitignore file on macOS

* Update yjit.rb

Co-authored-by: Randy Stauner <randy@r4s6.net>

* Use warn and always return nil if YJIT disasm not available.

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Co-authored-by: Randy Stauner <randy@r4s6.net>