[llvm-objcopy] Emit error if removing symtab referenced by group section

SHT_GROUP sections contain a reference to a symbol indicating their
"signature" symbol. The symbol table containing this symbol is referred
to by the group section's sh_link field. If llvm-objcopy is instructed
to remove the symbol table, it will emit an error.

This fixes https://bugs.llvm.org/show_bug.cgi?id=46153.

Reviewed By: jhenderson, Higuoxing

Differential Revision: https://reviews.llvm.org/D82274