perf stat: Refactor the code to strip csv output with ltrim()

perf stat: Refactor the code to strip csv output with ltrim()

To strip csv output, use ltrim() instead of just while loop and
isspace() at print_metric_{only}_csv().

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1491575061-704-3-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>