perf docs: Allow man page date to be specified

With this change if a perf_date parameter is provided to asciidoc then
it will override the default date written to the man page metadata.

Without this change, or if the perf_date isn't specified, then the
current date is written to the metadata.

Having this parameter allows the metadata to be constant if builds
happen on different dates.

The name of the parameter is intended to be consistent with the existing
perf_version parameter.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20190921041327.155054-1-irogers@google.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>