x64: add lfence accessor

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
pvclock: add LFENCE before reading the time stamp counter

The RDTSC instruction is unordered, ensure that the processor orders it
with an LFENCE instruction.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
tests: add monotonic clock test case

Ensure the clock doesn't go backwards.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>