glsl: only expose int64 atomics when extension is enabled

glsl: only expose int64 atomics when extension is enabled

This limits the exposure of these functions to when the extension is
available. Prevents crashes otherwise, as the rest of the infrastructure
doesn't necessarily expect these functions when the extension is not
available.

Fixes: 40c1f9883e5 ("mesa,glsl: add support for GL_NV_shader_atomic_int64")
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8533>