build: Add "fuzzing" as a valid cfg(..) attribute

The compiler is now able to warn if an invalid attribute (e.g like a
feature) is not available.

See https://blog.rust-lang.org/2024/05/06/check-cfg

build: Add "fuzzing" as a valid cfg(..) attribute

The compiler is now able to warn if an invalid attribute (e.g like a
feature) is not available.

See https://blog.rust-lang.org/2024/05/06/check-cfg.html for more
details.

Add build.rs files in the crates that use #cfg(fuzzing) to add fuzzing
to the list of valid cfg attributes.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

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