Rust fixes for v6.12 (2nd)

Toolchain and infrastructure:

 - Fix several issues with the 'rustc-option' macro. It includes a
   refactor from Masahiro of three '{cc,rust}-*' macros, which is not
   a fix but avoids repeating the same commands (which would be several
   lines in the case of 'rustc-option').

 - Fix conditions for 'CONFIG_HAVE_CFI_ICALL_NORMALIZE_INTEGERS'. It
   includes the addition of 'CONFIG_RUSTC_LLVM_VERSION', which is not a
   fix but is needed for the actual fix.

And a trivial grammar fix.
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