arch: arm: cortex-r: Add compiler tuning for Cortex-R82

Change the GCC toolchain configuration to make use of the Cortex-R82
target. When Cortex-R82 was added as a GCC toolchain option, the GCC
version of the Zephyr SDK did not support Cortex-R82 tuning. Zephyr was
therefore compiled compiled for the Armv8.4-A architecture. Since Zephyr
SDK 0.15.0 (which updated GCC from 10.3.0 to 12.1.0) coupled with Zephyr
3.2, the Cortex-R82 target is supported.

The Armv8-R AArch64 architecture does not support the EL3 exception level.
EL3 support is therefore made conditional on Armv8-R vs Armv8-A.

Signed-off-by: Debbie Martin <Debbie.Martin@arm.com>