[llvm] [Offload][cmake] Add GPU test job limit for AMDGPU buildbot cmake cache (PR #146611)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 approved this pull request.

Still not entirely sure this is safe, but it's worth a shot. Maybe they fixed some HSA bugs since I last checked.

https://github.com/llvm/llvm-project/pull/146611