x86/barriers, kcsan: Use generic instrumentation for non-smp barriers

Prefix all barriers with __, now that asm-generic/barriers.h supports
defining the final instrumented version of these barriers. The change is
limited to barriers used by x86-64.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>