[llvm-strip] Allow copying relocation sections without symbol tables.

Summary:
Fixes the error "Link field value 0 in section .rela.plt is invalid" when copying/stripping certain binaries. Minimal repro:

```
$ cat /tmp/a.c
int main() { return 0; }
$ clang -static /tmp/a.c -o /tmp/a
$ llvm-strip /tmp/a -o /tmp/b
llvm-strip: error: Link field value 0 in section .rela.plt is invalid.
```

Reviewers: jakehehrlich, alexshap

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51493

llvm-svn: 341419