Restore some ABI compatibility with Julia 1.6 (#40851)
In Julia <= 1.6, the C API function `jl_init` and `jl_init_with_image` were
exported on the ABI level under the names `jl_init__threading` and
`jl_init_with_image__threading`. This was changed during development of Julia
1.7, but unfortunately that broke ABI compatibility with binaries build
against Julia <= 1.6.
Add back and export two simple wrapper functions to restore this aspect of ABI
compatibility.
