perf Documentation: Support for asciidoctor

The asciidoc package seems behind the recent big wave of python3
conversion, and we were advised to switch to asciidoctor instead. It's
almost compatible but some extensions used for perf documentation don't
work with it. Here is the patch to cover them, and add the proper
support for asciidoctor.

Pass USE_ASCIIDOCTOR=yes to make for using asciidoctor instead of
asciidoc. The man source and manual attributes are passed via command
options. The support for these attributes have been fixed in the
latest asciidoctor code.

Since asciidoctor can covert to a man page and an HTML directly, we
can omit the dependency on xmlto when USE_ASCIIDOCTOR is set.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180424150456.17353-1-tiwai@suse.de
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>