How are modern processor instructions exposed to userland?

[Mayuresh Kathe]

Hello,

I have been wondering about this, and couldn't find an answer on the Web 
because I didn't know what to search for.

Let's think about the AVX-512 instruction introduced with the latest 
Intel Core i9 Skylake-X processor.
I wanted to know how one could use such capabilities via regular C.

There should be some way, someone working on the FreeBSD core who would 
expose that instruction capability to the userland via libc, right?
If my assumption is correct, may I know who does that? And, how do they 
go about doing it?
Obviously, knowledge of x86-64 assembler would be a must, so would 
knowledge of C89; right?

Thanks,

~Mayuresh