docs: programming-languages: refresh blurb on clang support

Building the kernel with Clang doesn't rely on third party patches, and
has not for a few years now.

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://lore.kernel.org/r/20200929211936.580805-1-ndesaulniers@google.com


[jc: Took out duplicated "docs" pointed out by Randy]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>