Documentation/llvm: refresh docs

Recent fixes for an embargoed hardware security vulnerability failed to
link with ld.lld (LLVM's linker).  [0]  To be fair, our documentation
mentions ``CC=clang`` foremost with ``LLVM=1`` being buried "below the
fold."

We want to encourage the use of ``LLVM=1`` rather than just
``CC=clang``. Make that suggestion "above the fold" and "front and
center" in our docs.

While here, the following additional changes were made:
- remove the bit about CROSS_COMPILE setting --target=, that's no longer
  true.
- Add ARCH=loongarch to the list of maintained targets (though we're
  still working on getting defconfig building cleanly at the moment;
  we're pretty close).
- Bump ARCH=powerpc from CC=clang to LLVM=1 status.
- Promote ARCH=riscv from being Maintained to being Supported. Android
  is working towards supporting RISC-V, and we have excellent support
  from multiple companies in this regard.
- Note that the toolchain distribution on kernel.org has been built with
  profile data from kernel builds.
- Note how to use ccache with clang.

Link: https://github.com/ClangBuiltLinux/linux/issues/1907

 [0]
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>