Export offsets necessary for external codegen (#49548)

(cherry picked from commit faced27)

Author: vchuravy

src/init.c

@@ -733,6 +733,11 @@ JL_DLLEXPORT void julia_init(JL_IMAGE_SEARCH rel)
     jl_init_intrinsic_properties();
 
     jl_page_size = jl_getpagesize();
+
+    // Important offset for external codegen.
+    jl_task_gcstack_offset = offsetof(jl_task_t, gcstack);
+    jl_task_ptls_offset = offsetof(jl_task_t, ptls);
+
     jl_prep_sanitizers();
     void *stack_lo, *stack_hi;
     jl_init_stack_limits(1, &stack_lo, &stack_hi);

src/jl_exported_data.inc

@@ -133,5 +133,7 @@
     XX(jl_n_threadpools, int) \
     XX(jl_n_threads, _Atomic(int)) \
     XX(jl_options, jl_options_t) \
+    XX(jl_task_gcstack_offset, int) \
+    XX(jl_task_ptls_offset, int) \
 
 // end of file

src/julia.h

@@ -1970,6 +1970,9 @@ JL_DLLEXPORT void JL_NORETURN jl_no_exc_handler(jl_value_t *e, jl_task_t *ct);
 JL_DLLEXPORT JL_CONST_FUNC jl_gcframe_t **(jl_get_pgcstack)(void) JL_GLOBALLY_ROOTED JL_NOTSAFEPOINT;
 #define jl_current_task (container_of(jl_get_pgcstack(), jl_task_t, gcstack))
 
+extern JL_DLLIMPORT int jl_task_gcstack_offset;
+extern JL_DLLIMPORT int jl_task_ptls_offset;
+
 #include "julia_locks.h"   // requires jl_task_t definition
 
 JL_DLLEXPORT void jl_enter_handler(jl_handler_t *eh);