[PATCH v2 resent] msvcrt: Add an ermsb-based memset for x86/64.

[Piotr Caban]

Hi Guillaume,

The "rep stosb" can't be used unconditionally since it's very slow on 
some processors.

Rémi sent a similar patch that adds the checks, see:
https://source.winehq.org/patches/data/214107
He also improved the C version of the function:
https://source.winehq.org/patches/data/214106

Is the "improved C" version good enough for your use case?

Thanks,
Piotr