[ARM][AArch64] Add Cortex-A78C Support for Clang and LLVM

This patch upstreams support for the Armv8-a Cortex-A78C
processor for AArch64 and ARM.

In detail:

Adding cortex-a78c as cpu option for aarch64 and arm targets in clang
Adding Cortex-A78C CPU name and ProcessorModel in llvm
Details of the CPU can be found here:
https://www.arm.com/products/silicon-ip-cpu/cortex-a/cortex-a78c