Document asm_cfg inline assembly attributes

[PLeVasseur]

Summary

• document asm_cfg attribute support on inline assembly templates/operands and update the inline-assembly grammar
• add attribute legality rules and an example under the asm macros section
• record syntax/paragraph updates in the Rust 1.93.0 changelog entry
• Closes [Change]: [1.93] Stabilize asm_cfg #647

Reference alignment

• Reference PR: document cfg conditions on inline assembly templates and operands reference#2063
• Reference sections: https://doc.rust-lang.org/reference/inline-assembly.html#syntax, https://doc.rust-lang.org/reference/inline-assembly.html#attributes
• Grammar: Reference introduces AsmAttrFormatString/AsmAttrOperand; FLS introduces AssemblyTemplate/AssemblyAttribute* wrappers to match, splitting operands into RegisterArgument, AbiClobber, and AssemblyOption
• Legality: cfg/cfg_attr support and the template-first rule match the Reference; FLS makes cfg-false argument behavior explicit to align with conditional compilation semantics
• Reference note about rustc attribute handling is omitted because it is implementation detail without semantic effect

Testing

• ./make.py

[PLeVasseur]

Hey @tshepang, @kirtchev-adacore -- could this be reviewed?

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.