armv8rng: Don't require toolchain to support FEAT_RNG

We have the mechanism in place to support encoding system registers
explicitly, so use that rather than requiring LLVM 13+, which breaks our
current set of GitHub CI builds.

Fixes: 9eecef052155 ("Add an Armv8 rndr random number provider")
(cherry picked from commit 9560ac4b638edf688566f576adc65d3654f2240c)