perf bench: Add malloc micro-benchmark

The new 'perf bench mem malloc' allocates and frees given size of
memory region (default: 100MB) with single thread and multiple
threads.  It runs two tests - one for allococating and immediate
free-ing the memory (something like "free(malloc(size));") and another
for allocating memory region first and then free-ing the region at
last.

An example output follows:

  $ perf bench mem malloc
  # Running mem/malloc benchmark...
  # Allocating 100MB Bytes in single thread...

       108.146456 GB/Sec	 (immediate free)
       730.919350 MB/Sec	 (last free)

  # Allocating 100MB Bytes for each 12 thread...

        63.413042 GB/Sec	 (immediate free)
        15.943981 MB/Sec	 (last free)

Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>