Switch all of the sample code to use the function names from
tools/lib/bpf so that they're consistent with that, and to declare their
own log buffers. This allow the next commit to be purely devoted to
getting rid of the duplicate library in samples/bpf.

Committer notes:

Testing it:

On a fedora rawhide container, with clang/llvm 3.9, sharing the host
linux kernel git tree:

  # make O=/tmp/build/linux/ headers_install
  # make O=/tmp/build/linux -C samples/bpf/

Since I forgot to make it privileged, just tested it outside the
container, using what it generated:

  # uname -a
  Linux jouet 4.9.0-rc8+ #1 SMP Mon Dec 12 11:20:49 BRT 2016 x86_64 x86_64 x86_64 GNU/Linux
  # cd /var/lib/docker/devicemapper/mnt/c43e09a53ff56c86a07baf79847f00e2cc2a17a1e2220e1adbf8cbc62734feda/rootfs/tmp/build/linux/samples/bpf/
  # ls -la offwaketime
  -rwxr-xr-x. 1 root root 24200 Dec 15 12:19 offwaketime
  # file offwaketime
  offwaketime: ELF 64-bit LSB executable, x86-64, versi...