Documentation: update with the new PowerPC support

  - To give details on support, add a "Constraints" column to the table.

  - To avoid listing all the architectures we support in the quick start,
    remove them. Note that this is anyhow explain in the top of the file,
    (with a reference to the arch support file).

    Take also the chance to explain more explicitly that
    `CC=clang` and `LLVM=1` are the way to go.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

Documentation/rust/arch-support.rst

@@ -12,12 +12,18 @@ Below is a general summary of architectures that currently work. Level of
 support corresponds to ``S`` values in the ``MAINTAINERS`` file.
 
 .. list-table::
-   :widths: 10 10
+   :widths: 10 10 10
    :header-rows: 1
 
    * - Architecture
      - Level of support
-   * - arm64
+     - Constraints
+   * - ``arm64``
      - Maintained
-   * - x86_64
+     - None
+   * - ``powerpc``
      - Maintained
+     - ``ppc64le`` only, ``RUST_OPT_LEVEL >= 2``, Clang only
+   * - ``x86``
+     - Maintained
+     - ``x86_64`` only

Documentation/rust/quick-start.rst

@@ -159,9 +159,17 @@ the example Rust driver ``Rust example`` (``CONFIG_RUST_EXAMPLE``).
 Building
 --------
 
-Building a x86_64 or arm64 kernel with either GCC, Clang or a complete LLVM
-toolchain should all work. However, please note that using GCC is more
-experimental at the moment.
+Building a kernel with Clang or a complete LLVM toolchain is the best supported
+setup at the moment. That is::
+
+    make ARCH=... CROSS_COMPILE=... CC=clang -j...
+
+or::
+
+    make ARCH=... CROSS_COMPILE=... LLVM=1 -j...
+
+Using GCC also works for some configurations, but it is *very* experimental at
+the moment.
 
 
 Hacking