Perform library linking after deferred codegen.

[maleadt]

I noticed a gpu_gc_pool_alloc validation error, where the allocation was only introduced in a deferred module. By moving library linking after codegen, we ensure that we'll link the necessary modules.

In addition, I now only perform the linking for the top-level compilation job, whereas we were linking it multiple times before.

cc @vchuravy, as Enzyme is a user of deferred codegen.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (5536c45) 82.58% compared to head (570d435) 82.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   82.58%   82.59%   +0.01%     
==========================================
  Files          23       23              
  Lines        3124     3126       +2     
==========================================
+ Hits         2580     2582       +2     
  Misses        544      544              

Impacted Files	Coverage Δ
src/driver.jl	93.29% <100.00%> (+0.07%)	⬆️

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