[llvm-objcopy] Fix crash when removing symbol table at same time as adding a symbol

This patch resolves crash that occurs when user wanted to remove all
symbols and add a brand new one using:

```
llvm-objcopy -R .symtab --add-symbol foo=1234 in.o out.o
```

Before these changes the symbol table internally being null when adding
new symbols. For now we will regenerate symtab in this case.

This fixes: https://bugs.llvm.org/show_bug.cgi?id=43930

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D82935