Rust fixes for v6.12 (3nd)

Toolchain and infrastructure:

 - Avoid build errors with old 'rustc's without LLVM patch version
   (important since it impacts people that do not even enable Rust).

 - Update LLVM version for 'HAVE_CFI_ICALL_NORMALIZE_INTEGERS' in
   'depends on' condition (the fix was eventually backported rather than
   land in LLVM 19).
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