Add safety sections to xsave-family save intrinsics

[kiana1kaslana]

_xsave, _xsaveopt, _xsavec, _xsaves and their 64-bit variants are declared unsafe but carry no # Safety section. This adds the missing sections.

The size of the XSAVE area these instructions write depends on both save_mask, which selects the features to save, and XCR0, the extended control register in which the OS records the features it has enabled. The safety sections state this relationship and require mem_addr to be valid for writes of the whole area, as well as 64-byte aligned.

Doc-only change.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @sayantn (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @Amanieu, @adamgemmell, @davidtwco, @folkertdev, @sayantn
• @Amanieu, @adamgemmell, @davidtwco, @folkertdev, @sayantn expanded to Amanieu, adamgemmell, davidtwco, folkertdev, sayantn
• Random selection from Amanieu, adamgemmell, davidtwco, folkertdev, sayantn

[sayantn]

#2255

@rustbot reroll