Add missing BUILD file for //compiler_gym/third_party/programl.

[ChrisCummins]

Fixes #623.

[codecov-commenter]

Codecov Report

Merging #629 (bc02e8e) into development (c5b9289) will decrease coverage by 0.25%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development     #629      +/-   ##
===============================================
- Coverage        88.75%   88.50%   -0.26%     
===============================================
  Files              115      115              
  Lines             7019     7019              
===============================================
- Hits              6230     6212      -18     
- Misses             789      807      +18     

Impacted Files	Coverage Δ
...er_gym/third_party/inst2vec/inst2vec_preprocess.py	69.73% <0.00%> (-4.22%)	⬇️
compiler_gym/util/commands.py	83.33% <0.00%> (-2.39%)	⬇️
compiler_gym/envs/llvm/datasets/cbench.py	79.42% <0.00%> (-1.09%)	⬇️
...loop_tool/service/loop_tool_compilation_session.py	89.79% <0.00%> (-0.69%)	⬇️
compiler_gym/envs/compiler_env.py	91.85% <0.00%> (-0.50%)	⬇️

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