[ARM] Add Cortex-A78 and Cortex-X1 Support for Clang and LLVM

This patch upstreams support for the Arm-v8 Cortex-A78 and Cortex-X1
processors for AArch64 and ARM.

In detail:
- Adding cortex-a78 and cortex-x1 as cpu options for aarch64 and arm targets in clang
- Adding Cortex-A78 and Cortex-X1 CPU names and ProcessorModels in llvm

details of the CPU can be found here:
https://www.arm.com/products/cortex-x

https://www.arm.com/products/silicon-ip-cpu/cortex-a/cortex-a78

The following people contributed to this patch:
- Luke Geeson
- Mikhail Maltsev

Reviewers: t.p.northover, dmgreen

Reviewed By: dmgreen

Subscribers: dmgreen, kristof.beyls, hiraditya, danielkiss, cfe-commits,
llvm-commits, miyuki

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D83206