Ruby

Add String#getbyte YJIT implementation (#8397)

• 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 [email protected]

• Skip the getbyte test for RJIT for now

Co-authored-by: John Hawthorn [email protected]
Co-authored-by: Takashi Kokubun [email protected]