[NFC] codegen: Move target ABI calculation into `jl_jit_abi_converter` (#58718)
Makes the API more directly reflect the operation here, which is
essentially "Give me a specptr with ABI `abi` that invokes CodeInstance
`codeinst`"
This relieves the caller of the responsibility to peek into the
CodeInstance and pull out all of the correct bits to determine the
appropriate callee ABI.