perf bench: Make the mem/memcpy tests more user-friendly

mem-memcpy.c uses perf event system calls to obtain CPU clocks.
And it suddenly dies with BUG_ON() when it running on Linux
doesn't support perf event.

Also fail at calloc() can occur easily when too large
length is passed. Fail of calloc() causes sudden death
with assert().

These behaviours are not friendly. So I fixed the treating of
errors.

Signed-off-by: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1258688237-3797-1-git-send-email-mitake@dcl.info.waseda.ac.jp>
[ v2: improved a few small details ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>