rust: cfi: fix `patchable-function-entry` starting version

rust: cfi: fix `patchable-function-entry` starting version

The `-Zpatchable-function-entry` flag is available since Rust
1.81.0, not Rust 1.80.0, i.e. commit ac7595fdb1ee ("Support for -Z
patchable-function-entry") in upstream Rust.

Fixes: ca627e636551 ("rust: cfi: add support for CFI_CLANG with Rust")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Link: https://lore.kernel.org/r/20240925141944.277936-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>