[llvm-objcopy] Default --output-target to --input-target when unspecified

Fixes PR42171.

In GNU objcopy, if -O (--output-target) is not specified, the value is
copied from -I (--input-target).

```
objcopy -I binary -B i386:x86-64 a.txt b       # b is copied from a.txt
llvm-objcopy -I binary -B i386:x86-64 a.txt b  # b is an x86-64 object file
```

This patch changes our behavior to match GNU. With this change, we can
delete code related to -B handling (D67215).

Reviewed By: jakehehrlich

Differential Revision: https://reviews.llvm.org/D67144

llvm-svn: 371913