RV64GC cores is a really vague definition of the cores.
SiFive U74 21G1 is a way more precise definition. It contains many extensions to mere RV64GC.
And it implements not just the 2019 version of the unprivileged spec but also a version of the privileged one. This is not implied by merely stating "RV64GC".
The specification for SiFive's U74 21G1 version[0] will tell you about the details of the supported extensions.
To save you the work of interpreting the document, I'll tell you it is actually Zba and Zbb, while missing the Zbc and Zbs extensions.
Zbb being the one that has CPOP and related instructions, thus JH7110 has them.
https://www.cnx-software.com/2022/08/29/starfive-jh7110-risc...
https://doc-en.rvspace.org/JH7110/PDF/JH7110_Datasheet.pdf
Where do you find B-extension instructions documented?
Usually we want it citing at least an RVA22U64 profile, which I do not find anywhere in these materials.