The ld/macho code currently understands a subset of the mach-o load
commands. I've encountered one of these in the wild in a Go-produced
binary, which tripped up the Go linker because its switch statement
expects its list of load commands to be exhaustive; the rest I've
added for the sake of completion.

The ruby-macho library is a good non-Darwin header resource for these:
https://github.com/homebrew/ruby-macho

Fixes #25908

Change-Id: Ib54c065d27e87d8726a9870df05a2bae24828b98
GitHub-Last-Rev: 655e3f488a4c1a89896a40edb0e1f715a5d3f734
GitHub-Pull-Request: golang/go#25906
Reviewed-on: https://go-review.googlesource.com/119115Reviewed-by: Ian Lance Taylor <iant@golang.org>