Clarify interaction of asm-goto with IBT

[nbdd0121]

Document the restriction that the asm-goto labels should not be jumped to indirectly if there's an architectural distinction (e.g. x64 with IBT).

This restriction is in place for rustc_codegen_gcc, and for LLVM, it's currently allowed, but https://discourse.llvm.org/t/rfc-asm-goto-vs-branch-target-enforcement/85845 may disallow this in the future.

This is to clarify rust-lang/rust#119364 (comment). cc @programmerjake

[traviscross]

@rfcbot fcp merge

[rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[joshtriplett]

This looks reasonable. Hopefully in the future there will be a label indirect mechanism or similar.

@rfcbot reviewed

[nikomatsakis]

@rfcbot reviewed

[tmandry]

@rfcbot reviewed

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

psst @traviscross, I wasn't able to add the final-comment-period label, please do so.