Introduce _CMM_TOOLCHAIN_SUPPORT_C11_MM

Introduce _CMM_TOOLCHAIN_SUPPORT_C11_MM

If the toolchain used supports the C11 memory model, define the private
_CMM_TOOLCHAIN_SUPPORT_C11_MM macro.

This is used to implement `uatomic_load()/uatomic_store' in term of
`__atomic' builtins instead of using volatile accesses, therefore
reducing possible redundant memory barriers for some memory orders.

Change-Id: I0df3c202daf74ac012338583b761de15687d25f3
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>