[libc] Add document for a recipe to do a full standalone cross-build.

Reviewed By: jeffbailey

Differential Revision: https://reviews.llvm.org/D141037

libc/docs/full_cross_build.rst

diff --git a/libc/docs/full_cross_build.rst b/libc/docs/full_cross_build.rst
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+.. _full_cross_build:
+
+================
+Full Cross Build
+================
+
+.. contents:: Table of Contents
+   :depth: 1
+   :local:
+
+In this document, we will present a recipe to cross build a full libc. When we
+say *cross build* a full libc, we mean that we will build the libc for a target
+system which is not the same as the system on which the libc is being built.
+For example, you could be building for a bare metal aarch64 *target* on a Linux
+x86_64 *host*.
+
+Configure the full cross build of the libc
+==========================================
+
+Below is a simple recipe to configure the libc for a cross build.
+
+.. code-block:: sh
+
+  $> cd llvm-project  # The llvm-project checkout
+  $> mkdir build
+  $> cd build
+  $> cmake ../llvm  \
+     -G Ninja \ # Generator
+     -DLLVM_ENABLE_PROJECTS=libc  \ # Enable the libc project
+     -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++  \
+     -DLLVM_LIBC_FULL_BUILD=ON   \ # We are building the full libc
+     -DLIBC_TARGET_TRIPLE=<Your target triple>
+
+We will go over the special options passed to the ``cmake`` command above.
+
+* **Enabled Projects** - Since we want to build the libc project, we list
+  ``libc`` as the enabled project.
+* **The full build option** - Since we want to build the full libc, we pass
+  ``-DLLVM_LIBC_FULL_BUILD=ON``.
+* **The target triple** - This is the target triple of the target for which
+  we are building the libc. For example, for a Linux 32-bit Arm target,
+  one can specify it as ``arm-linux-eabi``.
+
+Build and install
+=================
+
+After configuring the build with the above ``cmake`` command, one can build the
+the libc for the target with the following command:
+
+.. code-block:: sh
+   
+   $> ninja libc
+
+The above ``ninja`` command will build the ``libc.a`` static archive for the
+target specified with ``-DLIBC_TARGET_TRIPLE`` to the ``cmake`` command.
+
+Building for bare metal
+=======================
+
+To build for bare metal, all one has to do is to specify the
+`system <https://clang.llvm.org/docs/CrossCompilation.html#target-triple>`_
+component of the target triple as ``none``. For example, to build for a
+32-bit arm target on bare metal, one can use a target triple like
+``arm-none-eabi``.