Add the `memory_scope_all_devices` enum value, which is restricted to
OpenCL 3.0 or newer and the `__opencl_c_atomic_scope_all_devices`
feature. Also guard `memory_scope_all_svm_devices` accordingly, which
is already available in OpenCL 2.0.
The `__opencl_c_atomic_scope_all_devices` feature is header-only, so
set its define to 1 in `opencl-c-base.h`. This is done
unconditionally at the moment, as the mechanism for disabling
header-only options hasn't been decided yet.
This patch only adds a negative test for now. Ideally adding a CL3.0
run line to `atomic-ops.cl` should suffice as a positive test, but we
cannot do that yet until (at least) generic address spaces and program
scope variables are supported in OpenCL 3.0 mode.