Restore some ABI compatibility with Julia 1.6 (JuliaLang#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.

src/jl_exported_funcs.inc

@@ -275,6 +275,8 @@
     XX(jl_init_restored_modules) \
     XX(jl_init) \
     XX(jl_init_with_image) \
+    XX(jl_init__threading) \
+    XX(jl_init_with_image__threading) \
     XX(jl_install_sigint_handler) \
     XX(jl_instantiate_type_in_env) \
     XX(jl_instantiate_unionall) \

src/jlapi.c

@@ -91,6 +91,19 @@ JL_DLLEXPORT void jl_init(void)
     free(libbindir);
 }
 
+// HACK: remove this for Julia 1.8 (see <https://github.com/JuliaLang/julia/issues/40730>)
+JL_DLLEXPORT void jl_init__threading(void)
+{
+    jl_init();
+}
+
+// HACK: remove this for Julia 1.8 (see <https://github.com/JuliaLang/julia/issues/40730>)
+JL_DLLEXPORT void jl_init_with_image__threading(const char *julia_bindir,
+                                     const char *image_relative_path)
+{
+    jl_init_with_image(julia_bindir, image_relative_path);
+}
+
 JL_DLLEXPORT jl_value_t *jl_eval_string(const char *str)
 {
     jl_value_t *r;