Ensure testing for `bitreverse` when support in CBMC is available

[adpaco-aws]

Soon we will add partial support for the bitreverse intrinsic in Kani. CBMC does not support it but is being implemented in diffblue/cbmc#6581

When support is available, fix any issues with our implementation and enable testing.

[tautschnig]

diffblue/cbmc#6638 has been issued, so a release that includes support for bitreverse should be out before the end of the day.