Auto merge of rust-lang#3752 - Kixunil:simd-sha256, r=RalfJung

Implement SHA256 SIMD intrinsics on x86

Disclaimer: this is my first contribution to `miri`'s code. It's quite possible I'm missing something. This code works but may not be the cleanest/best possible.

It'd be useful to be able to verify code implementing SHA256 using SIMD since such code is a bit more complicated and at some points requires use of pointers. Until now `miri` didn't support x86 SHA256 intrinsics. This commit implements them.

Author: bors