timer: add precise TSC function

timer: add precise TSC function

According to Intel Developer's Manual:

"The RDTSC instruction is not a serializing instruction. It does not necessarily wait
 until all previous instructions have been executed before reading the counter. Simi-
 larly, subsequent instructions may begin execution before the read operation is
 performed. If software requires RDTSC to be executed only after all previous instruc-
 tions have completed locally, it can either use RDTSCP (if the processor supports that
 instruction) or execute the sequence LFENCE;RDTSC."

So add a rte_rdtsc_precise function that do a memory barrier before rdtsc to
synchronize operations and ensure that the TSC read is done at the expected place.
Use r/w memory barrier instead of lfence to serialize both loads and stores.

Signed-off-by: Didier Pallard <didier.pallard@6wind.com>
Reviewed-by: François-Frédéric Ozog <ff@ozog.com>
Reviewed-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>