[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