pdl: Make primite enums repr(u64) again

This was accidentally lost in https://r.android.com/2507964. Our tests
didn’t catch this because we disable all Rust lints when compiling the
generated backend.

Tag: #feature
Bug: 228306436
Test: atest pdl_tests pdl_rust_generator_tests_{le,be} pdl_generated_files_compile
Change-Id: I5c0c2c9fa05069e9d91326ad8d6d2e740322fb75