perf tests: Fix tarpkg build test error output redirection

perf tests: Fix tarpkg build test error output redirection

So we need to trow away just stdout, leaving stderr to be caught by
the build tests infrastructure, so that we can see what went wrong
when the tarpkg build test fails:

  $ make -C tools/perf build-test
  make: Entering directory '/home/acme/git/linux/tools/perf'
  - tarpkg: ./tests/perf-targz-src-pkg .
  tests/make:302: recipe for target 'tarpkg' failed
  make[1]: *** [tarpkg] Error 2
  Makefile:102: recipe for target 'build-test' failed
  make: *** [build-test] Error 2
  make: Leaving directory '/home/acme/git/linux/tools/perf'
  $ cat tools/perf/tarpkg
  ./tests/perf-targz-src-pkg .
    PERF_VERSION = 4.5.g05f5ec
    PERF_VERSION = 4.5.g05f5ec
  In file included from bench/mem-memcpy-x86-64-asm.S:9:0:
  bench/../../../arch/x86/lib/memcpy_64.S:5:29: fatal error: asm/cpufeatures.h: No such file or directory
  compilation terminated.
  mv: cannot stat ‘bench/.mem-memcpy-x86-64-asm.o.tmp’: No such file or directory
  make[5]: *** [bench/mem-memcpy-x86-64-asm.o] Error 1
  make[5]: *** Waiting for unfinished jobs....
  make[4]: *** [bench] Error 2
  make[4]: *** Waiting for unfinished jobs....
  make[3]: *** [perf-in.o] Error 2
  make[3]: *** Waiting for unfinished jobs....
  make[2]: *** [all] Error 2
  $

So the test flow is:

1. Run: 'make -C tools/perf build-test'

2. One of its tests failed, in this case, the 'tarpkg' one

3. Look at what went wrong, by looking at the output of that test, in
   tools/perf/tarpkg

Admittedly, this should be shortcircuited to showing what went wrong directly
from the 'make build-test' step, but lets first fix this tarpkg one and the
problem it spotted, which should be fixed by adding some extra file to the
tools/perf/MANIFEST so that detached tarballs continue being self contained and
build successfully.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-ynld6egoxolmftcddpnd7oh6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>