[CUDA][HIP] Externalize device var in anonymous namespace

Device variables in an anonymous namespace may be
referenced by host code, therefore they need to
be externalized in a similar way as a static device
variables or kernels in an anonymous namespace.

Fixes: https://github.com/ROCm-Developer-Tools/HIP/issues/3246

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D152164