Documentation/llvm: Add a section about supported architectures

The most common question around building the Linux kernel with clang is
"does it work?" and the answer has always been "it depends on your
architecture, configuration, and LLVM version" with no hard answers for
users wanting to experiment. LLVM support has significantly improved
over the past couple of years, resulting in more architectures and
configurations supported, and continuous integration has made it easier
to see what works and what does not.

Add a section that goes over what architectures are supported in the
current kernel version, how they should be built (with just clang or the
LLVM utilities as well), and the level of support they receive. This
will make it easier for people to try out building their kernel with
LLVM and reporting issues that come about from it.

Suggested-by: Miguel Ojeda <[e-mail]>
Signed-off-by: Nathan Chancellor <[e-mail]>
Reviewed-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

Documentation/kbuild/llvm.rst

···
 Currently, the integrated assembler is disabled by default. You can pass
 ``LLVM_IAS=1`` to enable it.
 
+Supported Architectures
+-----------------------
+
+LLVM does not target all of the architectures that Linux supports and
+just because a target is supported in LLVM does not mean that the kernel
+will build or work without any issues. Below is a general summary of
+architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level
+of support corresponds to "S" values in the MAINTAINERS files. If an
+architecture is not present, it either means that LLVM does not target
+it or there are known issues. Using the latest stable version of LLVM or
+even the development tree will generally yield the best results.
+An architecture's ``defconfig`` is generally expected to work well,
+certain configurations may have problems that have not been uncovered
+yet. Bug reports are always welcome at the issue tracker below!
+
+.. list-table::
+   :widths: 10 10 10
+   :header-rows: 1
+
+   * - Architecture
+     - Level of support
+     - ``make`` command
+   * - arm
+     - Supported
+     - ``LLVM=1``
+   * - arm64
+     - Supported
+     - ``LLVM=1``
+   * - mips
+     - Maintained
+     - ``CC=clang``
+   * - powerpc
+     - Maintained
+     - ``CC=clang``
+   * - riscv
+     - Maintained
+     - ``CC=clang``
+   * - s390
+     - Maintained
+     - ``CC=clang``
+   * - x86
+     - Supported
+     - ``LLVM=1``
+
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