Enable users to map from GV to Julia value

[vchuravy]

Two fold motivation.

Currently, we perform the JIT favourite trick on all globals, we inline the runtime pointers, this makes the resulting code non-relocatable. A concrete example of that would be something like:

function f(x)
    if x === :x
    end
end

The memory location of :x changes upon session restart (as do other things like Types, Singletons), and for objects like this Julia performs a simple pointer comparison. So we can actually support this operation on the GPU.

Yet, we cannot cache the resulting kernel, or if we do, we will perform the wrong computation. So what we must do instead is to initialize the global after loading it from cache.

Now that is a tricky affair (backend dependent), we can do it on LLVM IR, I believe AMDGPU could also do it, the CPU backends can use something like JITDylib, but I don't think we can do it on the obj file from Nvidia, and I don't about other backends.

So we should likely mark compilations that use globals with runtime pointers as ineligible for caching.

The second motivation stems from Enzyme where we would like to have a mapping from global to Julia object, and it can be hard to figure out which globals are Julia objects and which ones are not.

[codecov]

Codecov Report

❌ Patch coverage is 93.18182% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.49%. Comparing base (531dccd) to head (acd969c).

Files with missing lines	Patch %	Lines
src/jlgen.jl	92.10%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
- Coverage   75.28%   74.49%   -0.79%     
==========================================
  Files          24       24              
  Lines        3670     3682      +12     
==========================================
- Hits         2763     2743      -20     
- Misses        907      939      +32     

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[vchuravy]

I am open to a better name

[vchuravy]

We should likely just give up on caching this for now if !isempty(gv_to_value)

For proper caching, we would want to create an IdDict in the current parent module and instead of saving values we would want to save "offsets/keys" into that IdDict so that Julia performs the pkgimg reolcation for us.

[vchuravy]

😱 For backwards compatibility we need to support the internal arraylist_t format.

[vchuravy]

So we don't necessarily need to delete the initializer here, but it makes the code a bit more consistent.