[ARM] Add Cortex-X1C Support for Clang and LLVM

This patch upstreams support for the Arm-v8 Cortex-X1C processor for AArch64 and
ARM.

For more information, see:
- https://community.arm.com/arm-community-blogs/b/announcements/posts/arm-cortex-x1c
- https://developer.arm.com/documentation/101968/0002/Functional-description/Technical-overview/Components

The following people contributed to this patch:
- Simon Tatham
- Ties Stuij

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D117202