Talking futher to the person doing the glibc risc-v ifunc work, they
clarified that glibc _is_ passing hwcap as the first argument, and the
null pointer is actually the second argument.
https://sourceware.org/pipermail/libc-alpha/2023-August/150967.html
So since our whole purpose here was source compatibility, let's do what
they're actually doing, and let's add some tests. I've also added a test
that __riscv_hwprobe() works from an ifunc resolver because that's one
place where it might well be used. That said, one other thing that came
out of the discussion is that I actually went away and looked at a
sample of top apps to see how many are using ifuncs currently. The
result? Zero. So although this _might_ be interesting long term
(especially if clang gets riscv64 FMV), I think we've done more than we
need to with riscv64 ifuncs for now!
Test: ran locally, both dynamic and static tests
Change-Id: Ie2044d9f4e47c32c00ad381f045c537f4df38b08
620a72252e