[bpf] add relocation support

  . there should be no runtime relocation inside the bpf function.
  . relocation supported here mostly for debugging.
  . a test case is added.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
llvm-svn: 302055