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
Fangrui Song