[ARM] Add Cortex-A77 Support for Clang and LLVM

This patch upstreams support for the Arm-v8 Cortex-A77
processor for AArch64 and ARM.

In detail:
- Adding cortex-a77 as a cpu option for aarch64 and arm targets in clang
- Cortex-A77 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-a77

and a similar submission to GCC can be found here:
https://github.com/gcc-mirror/gcc/commit/e0664b7a63ed8305e9f8539309df7fb3eb13babe

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

Reviewers: t.p.northover, dmgreen, ostannard, SjoerdMeijer

Reviewed By: dmgreen

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D82887