Add safety sections to fxsave/fxrstor intrinsics

[kiana1kaslana]

The _fxsave, _fxrstor, _fxsave64 and _fxrstor64 intrinsics are declared unsafe but carry no # Safety section, unlike most of the other intrinsics in this crate. This adds the missing sections.

_fxsave writes a 512-byte FXSAVE image to memory and _fxrstor reads one back, so the sections state the validity and 16-byte alignment requirements of mem_addr. For the restore side they additionally require the memory to hold a well-formed FXSAVE image written by a prior save, with a legal value in the MXCSR portion.

Doc-only change.

[rustbot]

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

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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
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