[PATCH v1 1/2] x86: Implement sched_yield syscall for x86 only.

[Florian Weimer]

* Noah Goldstein via Libc-alpha:

> We slightly optimize it by using `vzeroall` before the actual syscall.
> This returns the SSE, AVX, and ZMM_HI256 xsave/xrstor states to the
> init-state which allows the imminent context switch to skip
> saving/restoring those states.

Surely there is a better way to implement this, enabling something
similar for all system calls issued by libc on the kernel side?  It
changes userspace ABI, so it has to be opt-in.  Maybe it could be an
additional flag in the system call number, indicating that it is safe
to zap the vector state if it is beneficial.