rust: zephyr-sys: Fixup clang targets for RISCV

Rustc for RISCV encodes optional features on the CPU available as part
of the target tuple.  Clang, on the other hand does not.  In order to be
able to use libclang with bindgen on RISCV, we need to simplify the
target tuples a bit.  Do this by just matching 'riscv32' or 'riscv64'
and then passing in a generic tuple.  We aren't generating any code, and
the structs should always match between the targets.

Signed-off-by: David Brown <david.brown@linaro.org>