Simplify code for count intrinsics in next CBMC version

[adpaco-aws]

There are a couple of PRs that will add support for count builtins in CBMC. This will allow the count intrinsics code in RMC to be simplified and its regression tests to be enabled.

[adpaco-aws]

The unsupported tests are in

rust-tests/cbmc-reg/Count/Unstable/Cttz/fixme_main.rs
rust-tests/cbmc-reg/Count/Unstable/Cttz/fixme_main_fail.rs
rust-tests/cbmc-reg/Count/Unstable/Ctlz/fixme_main_fail.rs