perf bpf: Generalize bpf__setup_stdout()

perf bpf: Generalize bpf__setup_stdout()

We will use it to set up other bpf-output events, for instance to
generate augmented syscall entry tracepoints with pointer contents.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-4r7kw0nsyi4vyz6xm1tzx6a3@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>