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arch table in our docs that he refers to is outdated.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>