docs: programming-languages: refresh blurb on clang support

The kernel is written in the C programming language [c-language]_.

More precisely, the kernel is typically compiled with ``gcc`` [gcc]_

under ``-std=gnu89`` [gcc-c-dialect-options]_: the GNU dialect of ISO C90

(including some C99 features).

(including some C99 features). ``clang`` [clang]_ is also supported, see

docs on :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.

This dialect contains many extensions to the language [gnu-extensions]_,

and many of them are used within the kernel as a matter of course.

There is some support for compiling the kernel with ``clang`` [clang]_

and ``icc`` [icc]_ for several of the architectures, although at the time

of writing it is not completed, requiring third-party patches.

There is some support for compiling the kernel with ``icc`` [icc]_ for several

of the architectures, although at the time of writing it is not completed,

requiring third-party patches.

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