perf help: Use asprintf instead of adhoc equivalents

That doesn't chekcs malloc return and that, when using strbuf, if it
can't grow, just explodes away via die().

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-vr8qsjbwub7e892hpa9msz95@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>