[docs][llvm-objcopy] Write documentation for llvm-objcopy

This patch addresses https://bugs.llvm.org/show_bug.cgi?id=42183 by replacing
the stub markdown doc for llvm-objcopy with a full one describing the current
options available in llvm-objcopy.

Reviewed by: jakehehrlich, MaskRay

Differential Revision: https://reviews.llvm.org/D63820

llvm-svn: 365042