[AArch64] Add NVIDIA Carmel support

Summary:
NVIDIA's Carmel ARM64 cores are used in Tegra194 chips found in Jetson AGX Xavier, DRIVE AGX Xavier and DRIVE AGX Pegasus.

References:
* https://devblogs.nvidia.com/nvidia-jetson-agx-xavier-32-teraops-ai-robotics/#h.huq9xtg75a5e
* NVIDIA Xavier Series System-on-Chip Technical Reference Manual 1.3 (https://developer.nvidia.com/embedded/downloads#?search=Xavier%20Series%20SoC%20Technical%20Reference%20Manual)

Reviewers: sdesmalen, paquette

Reviewed By: sdesmalen

Subscribers: llvm-commits, ianshmean, kristof.beyls, hiraditya, jfb, danielkiss, cfe-commits, t.p.northover

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D77940