Move the fork_event event definition into libperf's event.h header
include.

In order to keep libperf simple, we switch 'u64/u32/u16/u8' types used
events to their generic '__u*' versions.

Perf added 'u*' types mainly to ease up printing __u64 values
as stated in the linux/types.h comment:

  /*
   * We define u64 as uint64_t for every architecture
   * so that we can print it with "%"PRIx64 without getting warnings.
   *
   * typedef __u64 u64;
   * typedef __s64 s64;
   */

Add and use new PRI_lu64 and PRI_lx64 macros for that.  Using extra '_'
to ease up the reading and differentiate them from standard PRI*64
macros.
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 <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190825181752.722-6-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>