Documentation:
https://www.khronos.org/registry/OpenCL/sdk/1.0/docs/man/xhtml/cl_khr_int64_base_atomics.html

Clang is aware of this extension:

tools/clang/lib/Basic/Targets.cpp:     
Opts.support("cl_khr_int64_base_atomics");
tools/clang/lib/Sema/Sema.cpp:      //   cl_khr_int64_base_atomics and
cl_khr_int64_extended_atomics
tools/clang/lib/Sema/Sema.cpp:      //   is supported and the
cl_khr_int64_base_atomics and
tools/clang/lib/Sema/Sema.cpp:      //   atomic_ptrdiff_t are supported if the
cl_khr_int64_base_atomics and
tools/clang/lib/Sema/Sema.cpp:            "cl_khr_int64_base_atomics
cl_khr_int64_extended_atomics");

BUFFER_ATOMIC_*_X2 and FLAT_ATOMIC_*_X2 are available in LLVM/AMDGPU.

User/potential test: bug 99764