* Add getbyte JIT implementation

Adds an implementation for String#getbyte for YJIT, along with a
bootstrap test. This should be helpful for pure Ruby implementations
and to avoid unneeded allocations.

Co-authored-by: John Hawthorn <jhawthorn@github.com>

* Skip the getbyte test for RJIT for now

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Co-authored-by: John Hawthorn <jhawthorn@github.com>
Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>