Export offsets necessary for external codegen

[vchuravy]

Enzyme performs external codegen that needs to access current_task and
the ptls. We have three options:

1. Hard-code it per Julia version/architecture
2. "Introspect" it by doing a code_llvm to get the offset
3. Expose a variable that Enzyme can read to get a stable answer.

We used to do 1. in the past which lead to fun crashes everytime it changed on the
Julia side.

We currently do 2., which leads to bad initialization times.

This is option 3.

[gbaraldi]

Is int fine here or should this be a ptrdiff_t?

[vchuravy]

Int should be fine. these are between 12-14