bpf: Fix tools and testing build.

I'm getting various build failures on sparc64.  The key is
usually that the userland tools get built 32-bit.

1) clock_gettime() is in librt, so that must be added to the link
   libraries.

2) "sizeof(x)" must be printed with "%Z" printf prefix.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>