GNU/Linux has historically had the following two prototypes:
1. Elf_Addr(uint64_t, void *)
2. Elf_Addr(uint64_t, void *, void *)
For the former, AT_HWCAP is passed in the first argument, and NULL in
the second. For the latter, AT_HWCAP is still passed, and the second
argument is a pointer to their home-grown __riscv_hwprobe function.
Should they want to use the third argument in future, they'll have to
introduce yet another prototype to allow for later expansion, and then
all users will have to check whether the second argument is NULL to know
if the third argument really exists. This is all rather silly and will
surely prove fun in the face of type-checking CFI.
Instead, be like arm64 and just define all 8 possible general purpose
register arguments up front. To naive source code that forgets non-Linux
OSes exist this will be compatible with prototype 1 above, since the
second argument will be 0 and it won't look further (though should we
start using the second argument for something that wouldn't be true any
more and it might think it's __riscv_hwprobe, but that incompatibility
is one we can defer committing to, and can choose to never adopt).
Until the standard interface for querying extension information[1] is
settled and implemented in FreeBSD there's not much you can do in a
resolver other than use HWCAP_ISA_B, but this gets the infrastructure in
place for when that day comes.
[1] https://github.com/riscv-non-isa/riscv-c-api-doc/pull/74