[AArch64] Add CPU Cortex-R82
This adds support for -mcpu=cortex-r82. Some more information about this
core can be found here:
One note about the system register: that is a bit of a refactoring because of
small differences between v8.4-A AArch64 and v8-R AArch64.
This is based on patches from Mark Murray and Mikhail Maltsev.
Differential Revision: https://reviews.llvm.org/D88660