perf build: Do not use -Wshadow on gcc < 4.8

As it is too strict, see https://lkml.org/lkml/2006/11/28/253 and
https://gcc.gnu.org/gcc-4.8/changes.html, that takes into account
Linus's comments (search for Wshadow) for the reasoning about -Wshadow
not being interesting before gcc 4.8.

Acked-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/r/20190719183417.GQ3624@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>