[llvm-objcopy] Fix sh_link

This diff fixes sh_link for various types of sections 
(i.e. for SHT_ARM_EXIDX, SHT_HASH). In particular, this change enables us
to use llvm-objcopy with clang -gsplit-dwarf for the target android-arm.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D45851

llvm-svn: 330478