Documentation/llvm: Update Supported Arch table

While watching Michael's new talk on Clang-built-Linux, I noticed the
arch table in our docs that he refers to is outdated.

Add hexagon and User Mode.  Bump MIPS and RISCV to LLVM=1.  PowerPC is
almost LLVM=1 capable; ppc64le works, but ppc64 (big endian) and ppc32
still need more work.

Link: https://youtu.be/W4zdEDpvR5c?t=399
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/llvm.rst

@@ -129,18 +129,24 @@ yet. Bug reports are always welcome at the issue tracker below!
    * - arm64
      - Supported
      - ``LLVM=1``
+   * - hexagon
+     - Maintained
+     - ``LLVM=1``
    * - mips
      - Maintained
-     - ``CC=clang``
+     - ``LLVM=1``
    * - powerpc
      - Maintained
      - ``CC=clang``
    * - riscv
      - Maintained
-     - ``CC=clang``
+     - ``LLVM=1``
    * - s390
      - Maintained
      - ``CC=clang``
+   * - um (User Mode)
+     - Maintained
+     - ``LLVM=1``
    * - x86
      - Supported
      - ``LLVM=1``