[CUDA][HIP] Fix gpu.used.external

Rename gpu.used.external as __clang_gpu_used_external as ptxas does not
allow . in global variable name.

Fixes: https://github.com/llvm/llvm-project/issues/54934

Reviewed by: Joseph Huber, Artem Belevich

Differential Revision: https://reviews.llvm.org/D123946