perf tool: simplify ARCH code in Makefile

The ARCH is first determined from `uname -m`, and then overridden to x86
anyway. This is unnecessarily ugly; follow the example set by
ffee0de (x86: Default to ARCH=x86 to avoid overriding CONFIG_64BIT,
2012-12-20) to simplify the code.

Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Michael Witten <mfwitten@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
perf tool: don't print bogus data on -e cycles

When only the cycles event is requested:

$ perf stat -e cycles dd if=/dev/zero of=/dev/null count=1000000
1000000+0 records in
1000000+0 records out
512000000 bytes (512 MB) copied, 0.26123 s, 2.0 GB/s

 Performance counter stats for 'dd if=/dev/zero of=/dev/null count=1000000':

       911,626,453 cycles                    #    0.000 GHz

       0.262113350 seconds time elapsed

The 0.000 GHz comment in the output is totally bogus and misleading. It
happens because update_shadow_stats() doesn't touch runtime_nsecs_stats;
it is only written when a requested counter matches a SW_TASK_CLOCK. In
our case, since we have only requested HW_CPU_CYCLES,
runtime_nsecs_stats is unavailable. So, omit printing the comment
altogether.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
perf tool: don't print bogus data on -e instructions

When only the instructions event is requested:

$ perf stat -e instructions git s
M  builtin-stat.c

 Performance counter stats for 'git s':

       917,453,420 instructions              #    0.00  insns per cycle

       0.213002926 seconds time elapsed

The 0.00 insns per cycle comment in the output is totally bogus and
misleading. It happens because update_shadow_stats() doesn't touch
runtime_cycles_stats when only the instructions event is requested. So,
omit printing the bogus data altogether.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
perf tool: simplify a git command in PERF-VERSION-GEN

Replace a complex git log invocation with a simpler git rev-parse
invocation.  If the command fails, we have larger problems: don't
squelch stderr output.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
perf tool: add example to perf-trace documentation

It's not clear that perf trace must be run as a privileged user, so
include an example demonstrating its usage with sudo. While at it,
update the synopsis to include options.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: David Ahern <dsahern@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>