[CUDA] Fix static device variables with -fgpu-rdc

NVPTX does not allow dots in the identifier, so ptxas errors out with
   fatal   : Parsing error near '.static': syntax error
because it parses .static as a directive. Avoid this problem by using
two underscores, similar to what OpenMP does for outlined functions.

Differential Revision: https://reviews.llvm.org/D108456