ROCmDiscovery: Ensure all globals are defined

[jpsamaroo]

This fixes a load-time error I get due to not having ROCm libraries available:

ERROR: InitError: UndefVarError: `libdevice_libs` not defined in `AMDGPU`
Suggestion: this global was defined as `AMDGPU.ROCmDiscovery.libdevice_libs` but not assigned a value.
Stacktrace:
  [1] functional(component::Symbol)
    @ AMDGPU ~/.julia/packages/AMDGPU/jLWP2/src/utils.jl:80
  [2] __init__()
    @ AMDGPU ~/.julia/packages/AMDGPU/jLWP2/src/AMDGPU.jl:210
...

Specifically, this doesn't remove the @error message that I get before this, but it just makes sure that using AMDGPU will still work without error even if the ROCm libraries aren't available.