Protection for the `xgetbv` instruction for systems that do not support
`xcr0` such as nehalem processors.
The `XSAVE` cpuid indicates support for `XSAVE`, `XRESTOR`, `XSETBV`,
`XGETBV` while `OSXSAVE` indicates if the operating system itself has
`XSAVE` turned on. Both must be checked at the same time.