perf bench: Rename 'mem-memcpy.c' => 'mem-functions.c'

So mem-memcpy.c started out as a simple memcpy() benchmark, then it grew
memset() functionality and now I plan to add string copy benchmarks as
well.

This makes the file name a misnomer: rename it to the more generic
mem-functions.c name.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1445241870-24854-5-git-send-email-mingo@kernel.org


[ The "rename" was introducing __unused, wasn't removing the old file,
  and didn't update tools/perf/bench/Build, fix it ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>