Implement support for getting hardware randomness on AArch64

Aarch64 is not currently supported. Here's some links on how we might support it.

• https://developer.arm.com/documentation/ddi0601/2020-12/AArch64-Registers/RNDRRS--Reseeded-Random-Number
• https://developer.arm.com/documentation/ddi0601/2024-09/AArch64-Registers/RNDR--Random-Number (gated on the feature bit).