[AArch64] FP/SIMD is not mandatory for v8-R (#79004)

The FP/SIMD instructions are optional for v8-R, so they should not be
marked as a dependency of HasV8_0rOps. This had the effect of disabling
some v8R-specific system registers when any of these features was
disabled.

I've moved these features to be enabled by default for Cortex-R82
(currently the only v8-R AArch64 core), matching the previous behavior,
and clang's default.

Based on a patch by Simi Pallipurath <simi.pallipurath@arm.com>