We need the 'page_size' variable in libperf, so move it there.

Add a libperf_init() as a global libperf init function to obtain this
value via sysconf() at tool start.

Committer notes:

Add internal/lib.h to tools/perf/ files using 'page_size', sometimes
replacing util.h with it if that was the only reason for having util.h
included.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lore.kernel.org/lkml/20190913132355.21634-33-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>