[BPF] generate R_BPF_NONE relocation for BTF DataSec variables

The variables in BTF DataSec type encode in-section offset.
R_BPF_NONE should be generated instead of R_BPF_64_32.

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D62460

llvm-svn: 361742