BACKPORT: Documentation/llvm: add documentation on building w/ Clang/LLVM

Added to kbuild documentation. Provides more official info on building
kernels with Clang and LLVM than our wiki.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
(cherry picked from commit fcf1b6a35c16ac500fa908a4022238e5d666eabf)
Change-Id: I33bc4a97cf9e9ef923712bb490df0bceac81dade

Documentation/kbuild/00-INDEX

@@ -1,5 +1,7 @@
 00-INDEX
 	- this file: info on the kernel build process
+llvm.txt
+	- how to compile Linux with llvm
 headers_install.txt
 	- how to export Linux headers for use by userspace
 kbuild.txt

Documentation/kbuild/llvm.txt

@@ -0,0 +1,83 @@
+Building Linux with Clang/LLVM
+==============================
+
+This document covers how to build the Linux kernel with Clang and LLVM
+utilities.
+
+About
+-----
+
+The Linux kernel has always traditionally been compiled with GNU
+toolchains such as GCC and binutils. Ongoing work has allowed for
+[Clang](https://clang.llvm.org/) and [LLVM](https://llvm.org/) utilities
+to be used as viable substitutes. Distributions such as
+[Android](https://www.android.com/),
+[ChromeOS](https://www.chromium.org/chromium-os), and
+[OpenMandriva](https://www.openmandriva.org/) use Clang built kernels.
+[LLVM is a collection of toolchain components implemented in terms of
+C++ objects](https://www.aosabook.org/en/llvm.html). Clang is a
+front-end to LLVM that supports C and the GNU C extensions required by
+the kernel, and is pronounced "klang," not "see-lang."
+
+Clang
+-----
+
+The compiler used can be swapped out via [CC=]{.title-ref} command line
+argument to [make]{.title-ref}. [CC=]{.title-ref} should be set when
+selecting a config and during a build.
+
+> make CC=clang defconfig
+>
+> make CC=clang
+
+Cross Compiling
+---------------
+
+A single Clang compiler binary will typically contain all supported
+backends, which can help simplify cross compiling.
+
+> ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
+
+[CROSS\_COMPILE]{.title-ref} is not used to prefix the Clang compiler
+binary, instead [CROSS\_COMPILE]{.title-ref} is used to set a command
+line flag: [--target <triple>]{.title-ref}. For example:
+
+> clang --target aarch64-linux-gnu foo.c
+
+LLVM Utilities
+--------------
+
+LLVM has substitutes for GNU binutils utilities. These can be invoked as
+additional parameters to [make]{.title-ref}.
+
+>    make CC=clang AS=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \\
+>     OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-objsize \\
+>     READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \\
+>     HOSTLD=ld.lld
+
+Getting Help
+------------
+
+-   [Website](https://clangbuiltlinux.github.io/)
+-   [Mailing
+    List](https://groups.google.com/forum/#!forum/clang-built-linux):
+    <clang-built-linux@googlegroups.com>
+-   [Issue Tracker](https://github.com/ClangBuiltLinux/linux/issues)
+-   IRC: \#clangbuiltlinux on chat.freenode.net
+-   [Telegram](https://t.me/ClangBuiltLinux): \@ClangBuiltLinux
+-   [Wiki](https://github.com/ClangBuiltLinux/linux/wiki)
+-   [Beginner
+    Bugs](https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22)
+
+Getting LLVM
+------------
+
+-   <http://releases.llvm.org/download.html>
+-   <https://github.com/llvm/llvm-project>
+-   <https://llvm.org/docs/GettingStarted.html>
+-   <https://llvm.org/docs/CMake.html>
+-   <https://apt.llvm.org/>
+-   <https://www.archlinux.org/packages/extra/x86_64/llvm/>
+-   <https://github.com/ClangBuiltLinux/tc-build>
+-   <https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source>
+-   <https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/>