For CPU/OS scheduling, use pthreads/OpenMP apis to set processor affinity for threads.
For SIMD, use compiler intrinsics.
Rather they rely on a mix of C compiler language extensions, inline or external Assembly written helpers functions, which any language compiled language also has available, when going out of the standard goes.
When most people say "I write in C", they don't mean abstract ISO C standard, with the possibility of CHAR_BIT=9. They mean "C for my machine" - so C with compiler extensions, assumptions about memory model, and yes, occasional inline assembly.
Other languages share the same features.
All apps using the above-mentioned are written in standard ANSI/ISO C. The implementation themselves are "system level" code and hence have Language/HW/OS specific extensions which is standard practice when interfacing with low-level code.
> any language compiled language also has available
In theory yes, but in practice never to the ease nor flexibility with which you can use C for the job. This is what people mean when they say "C is close to the metal" or "C is a high-level assembly language".
Proven before C was even a dream at AT&T, and by all other OS vendors outside Bell Labs using other systems languages.
Then people get to argue C can X, yeah provided it is the Compiler XYZ C dialect.
