perf tools: Move syscall number fallbacks from perf-sys.h to tools/arch/x86/include/asm/

And remove the empty tools/arch/x86/include/asm/unistd_{32,64}.h files
introduced by eae7a755

 ("perf tools, x86: Build perf on older
user-space as well").

This way we get closer to mirroring the kernel for cases where __NR_
can't be found for some include path/_GNU_SOURCE/whatever scenario.

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-kpj6m3mbjw82kg6krk2z529e@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>