add urcu/arch_generic.h

Most of the memory barrier definitions are shared between all
architectures, especially smp_* and mc/rmc/wmc.  Put them in
a common file.

[edit by Mathieu Desnoyers]

+ * arch_defaults.h: common definitions for multiple architectures.
 becomes
+ * arch_generic.h: common definitions for multiple architectures.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>