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

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 eae7a755ee81 ("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 <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>