We copy headers from include/, arch/ to allow tools/ use defines,
structs from newer kernels and still be able to build on older systems.

We then, as part of a build, check if those copies got out of sync, when
we emit a warning, so that we can check if something needs to be
reflected on the tools, e.g. a 'perf trace' syscall argument beautifier
needs tweaking.

But we don't have to be super strict with that, for instance, extra
spaces, tabs or blank lines aren't problematic, so change
check-headers.sh to have "--ignore-blank-lines --ignore-space-change" as
default "diff" arguments.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
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-d8emqpdc3m2qtzt1ei8ra2tf@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>