docs(changelog): note system ABI variadics already specified

[PLeVasseur]

Summary

• Define calling convention in the glossary and add a numbered rule for resolving extern "system" to the effective ABI (including the Windows x86_32 variadic exception).
• Add platform availability constraints for implementation-permission ABIs and specify extern block value-namespace placement.
• Restrict external function qualifiers to ItemSafety within unsafe extern blocks.
• Closes [Change]: [1.93] Stabilize declaration of C-style variadic functions for the system ABI rust-lang/fls#646

Reference alignment

• add 'system' to variadic ABIs rust-lang/reference#2069
• https://doc.rust-lang.org/reference/items/external-blocks.html#abi
• https://doc.rust-lang.org/reference/items/external-blocks.html#variadic-functions
• Note: 2024 requirement for unsafe extern blocks is intentionally omitted (FLS 2021).

Testing

• ./make.py