[llvm-objcopy][ELF] Preserve sh_link to .symtab when applicable

This change to llvm-objcopy preserves the ELF section sh_link to .symtab
so long as none of the symbol table indices have been changed.
Previously, any invocation of llvm-objcopy including a "no-op" would
clear any section sh_link to .symtab.

Differential Revision: https://reviews.llvm.org/D150859