update https://developer.android.com/ndk/guides/cpu-features

[enh-google]

https://developer.android.com/ndk/guides/cpu-features needs to add __riscv.

we might want to explicitly say that getauxval() isn't useful for riscv64. we might want to mention hwprobe, but should probably just emphasize that google/cpu_features is the better choice?

we should explicitly state that the ndk cpufeatures library doesn't include riscv64.