perf session: Make events_stats u64 to avoid overflow on 32-bit arches

Pekka Enberg reported weird percentages in perf report. It
turns out we are overflowing a 32-bit variables in struct
events_stats on 32-bit architectures.

Before:

 [acme@ana linux-2.6-tip]$ perf report -i pekka.perf.data 2> /dev/null | head -10
   281.96%       Xorg                        b710a561  [.] 0x000000b710a561
   140.15%       Xorg  [kernel]                        [k] __initramfs_end
    51.56%   metacity  libgobject-2.0.so.0.2000.1      [.] 0x00000000026e46
    35.12%  evolution  libcairo.so.2.10800.6           [.] 0x000000000203bd
    33.84%   metacity  libpthread-2.9.so               [.] 0x00000000007a3d

After:

 [acme@ana linux-2.6-tip]$ perf report -i pekka.perf.data 2> /dev/null | head -10
    30.04%       Xorg                       b710a561   [.] 0x000000b710a561
    14.93%       Xorg  [kernel]                        [k] __initramfs_end
     5.49%   metacity  libgobject-2.0.so.0.2000.1      [.] 0x00000000026e46
     ...