perf bench: Fix confused variable namings and descriptions in mem subsystem

perf bench: Fix confused variable namings and descriptions in mem subsystem

As Namhyung Kim pointed, there are confused namings and descriptions of words
"cycle" and "clock" in mem-memset.c and mem-memcpy.c.

With the option "-c" (or "--clock", now renamed as "--cycle"), mem subsystem
measures cost of memset() and memcpy() with cpu-cycles event.

But current mem subsystem source code contains lots of confused variable
namings and descriptions with "clock" (e.g. the variable use_clock). This is a
very bad style because there is another software event named "cpu-clock". This
patch replaces wrong usage of "clock" to "cycle".

v2: modified Documentation/perf-bench.txt for the descriptions of
--cycle option

Signed-off-by: Hitoshi Mitake <h.mitake@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1341236777-18457-1-git-send-email-h.mitake@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>