RISC-V: Base for complex extension implications

Thanks to the commit 48558a5e5471 ("RISC-V: Allow nested implications for
extensions"), we can write complex extension implications in theory.
However, to actually do that, we need to pass more information to
check_func.

For example, we want to imply 'Zcf' from 'F' if and only if the 'Zce'
extension is also enabled and XLEN is 32.  Passing rps is a way to
enable this.

This commit prepares for such complex extension implications.