This was historically the case, but no longer. Several architectures can be built with clang now and there's one that is built exclusively with clang (for lack of a gcc-based toolchain).

Also, Nick Desaulniers and team from Google work on fixes to the toolchains and kernel to keep it that way. [1]

[1] https://clangbuiltlinux.github.io/

This has not been true for some time, maybe 2017? Linux Foundation even had an LLVM Linux initiative. Building with Clang is supported

https://www.kernel.org/doc/html/v5.8/kbuild/llvm.html

> Certainly not the Linux kernel itself, that relies too heavily on GCC quirks.

True, but there was a large effort to clone in LLVM nearly all the GCC extensions and quirks that the Linux kernel requires, so that nowadays the Linux kernel can be mostly compiled also with LLVM. See https://bugs.llvm.org/show_bug.cgi?id=4068 and its long list of dependencies to get an idea of what had to be done.

Actually, I have a LLVM build kernel in a Debian server, with ThinLTO enabled config.

"Linux version 5.13.0+ (x@x) (Debian clang version 13.0.0-++20210418105309+a0898f0cecc7-1~exp1, LLD 13.0.0) #6 SMP Thu Jul 1 10:22:40 CDT 2021"