[docs][llvm-objcopy] Update --output-target text with right defaults

The --output-target documentation has slightly rotted, as the default is
no longer purely based on the input file format, but also the value of
--input-target. This patch updates the documentation to make this
explicit.

Reviewed by: MaskRay, alexshap

Differential Revision: https://reviews.llvm.org/D79318