[libclc] Enable 64bit atomic for NVPTX
cl_khr_int64_base_atomics and cl_khr_int64_extended_atomics are
enabled in intel/llvm clang/lib/Basic/Targets/NVPTX.h, but not in
upstream.
Enable it to fix test-e2e/WorkGroupMemory/reduction_lambda.cpp build.
This change will be upstreamed.
