Well... last time I had a look at the assembly code of syscall entry on x86_64, I was scared away... this piece of "assembly" does require some very niche C compiler options to be compatible (stack alignment and more if I recall properly).

Linux "C" code hard dependency on gcc/clang/("ultra complex compilers") is getting worse by the day. It should (very easy to say, I know) have stayed very simple and plain C99+ with smart macro definitions to be replaced with pure assembly or the missing bits for modern hardware programming (atomics/memory barriers, explicit unaligned access, etc), but those abominations like _generic (or static assert,__thread,etc) are toxic additions to the C standard (there are too many toxic additions and not enough removal/simplification/hardening in ISO C, yes, we will have to define a "C profile" which breaks backward compatibility with hardening and simplifications).

I don't say all extensions are bad, but I think they need more "smart and pertinent pick and choose" (and I know this is a tough call), just because they "cost". For instance, for a kernel, we know it must have fine grained control of ELF object sections... or we would get much more source files (one per pertinent section) or "many more source configuration macros" (....but there I start to wonder if it was not actually the "right" way instead of requiring a C compiler to support such extension, it moves everything to the linker script... which is "required" anyway for a kernel).

Linus T. is not omnipotent and can do only that much and a lot of "official" linux devs are putting really nasty SDK dependency requirements in everyday/everywhere kernels.

That said, on my side, many of my user apps are now directly using linux syscalls... but are written in RISC-V assembly interpreted on x86_64 (I have a super lean interpreter/syscall translater written in x86_64 assembly and a super lean executable format wrapped in ELF executable format), or very plain and simple C99+ (legacy or because I want some apps to be a bit more 'platform crossy'... for now).