Instead do the lookups just when creating the tracepoints, initially for
the most common, raw_syscalls:sys_{enter,exit}.

It works by having evsel->priv have a per tracepoint structure with
entries for the fields, for direct access, with the offset and a
function to get the value from the sample, doing the swap if needed.

Using a simple workload that does M millions write syscalls, we go from:

 # perf stat -i -e cycles /tmp/oldperf trace ./sc_hello 100 > /dev/null

 Performance counter stats for '/tmp/oldperf trace ./sc_hello 100':

     8,366,771,459 cycles

       2.668025928 seconds time elapsed

 # perf stat -i -e cycles perf trace ./sc_hello 100 > /dev/null

 Performance counter stats for 'perf trace ./sc_hello 100':

     8,345,187,650 cycles

       2.631748425 seconds time elapsed

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-eyfhvoo510a5i10b27dnvm88@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>