BPF: Add more relocation kinds

Currently, BPF only contains three relocations:
  R_BPF_NONE   for no relocation
  R_BPF_64_64  for LD_imm64 and normal 64-bit data relocation
  R_BPF_64_32  for call insn and normal 32-bit data relocation

Also .BTF and .BTF.ext sections contain symbols in allocated
program and data sections. These two sections reserved 32bit
space to hold the offset relative to the symbol's section.
When LLVM JIT is used, the LLVM ExecutionEngine RuntimeDyld
may attempt to resolve relocations for .BTF and .BTF.ext,
which we want to prevent. So we used R_BPF_NONE for such relocations.

This all works fine until when we try to do linking of
multiple objects.
  . R_BPF_64_64 handling of LD_imm64 vs. normal 64-bit data
    is different, so lld target->relocate() needs more context
    to do a correct job.
  . The same for R_BPF_64_32. More context is needed for
    lld target->relocate() to differentiate call insn vs.
    normal 32-bit data relocation.
  . Since relocations in .BTF and .BTF.ex...