[llvm-objcopy] Initial support for wasm in llvm-objcopy

Currently only supports simple copying, other operations to follow.

Reviewers: sbc100, alexshap, jhenderson

Differential Revision: https://reviews.llvm.org/D70930

This is a reland of a928d127 with a one-line fix to ensure that
the wasm version number is written as little-endian (it's the only
field in all of the binary format that's not a single byte or an
LEB, but we may have to watch out more when we start handling the
linking section).