barriers: Introduce barrier operations

Introduce a new API for barrier operations starting with a general
skeleton and the implementation for barrier_data_memory_fence_full().

Select a built-in or an arch-based implementation according to new
Kconfig symbols CONFIG_BARRIER_OPERATIONS_BUILTIN and
CONFIG_BARRIER_OPERATIONS_ARCH.

The built-in implementation falls back on the compiler built-in
function using __ATOMIC_SEQ_CST as it is done for the atomic APIs
already.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>