x86: Support RDTSCP for benchtests

RDTSCP waits until all previous instructions have executed and all
previous loads are globally visible before reading the counter.  RDTSC
doesn't wait until all previous instructions have been executed before
reading the counter.  All x86 processors since 2010 support RDTSCP
instruction.  This patch adds RDTSCP support to benchtests.

	* benchtests/Makefile (CPPFLAGS-nonlib): Add -DUSE_RDTSCP if
	USE_RDTSCP is defined.
	* sysdeps/x86/hp-timing.h (HP_TIMING_NOW): Use RDTSCP if
	USE_RDTSCP is defined.