[docs] Add "GNU binutils Replacements" section to command guide

Summary:
This splits out a section in the command guide for llvm tools that can be used as replacements for GNU tools. For pages that didn't exist, I added stub pages that can be individually filled in by followup patches.

Tested by running `ninja docs-llvm-html` and inspecting locally.

Reviewers: jhenderson, MaskRay, grimar, alexshap

Reviewed By: jhenderson, MaskRay, grimar

Subscribers: smeenai, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63014

llvm-svn: 363100