[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
diff --git a/llvm/docs/CommandGuide/llvm-size.md b/llvm/docs/CommandGuide/llvm-size.md
new file mode 100644
index 0000000..3952708
--- /dev/null
+++ b/llvm/docs/CommandGuide/llvm-size.md
@@ -0,0 +1,10 @@
+# llvm-size - print segment sizes
+
+## SYNOPSIS
+
+**llvm-size** [*options*]
+
+## DESCRIPTION
+
+**llvm-size** is a tool that prints segment sizes in object files. The goal is
+to make it a drop-in replacement for GNU's **size**.