[AArch64] Add Cortex-A34 Support for clang and llvm

This patch upstreams support for the AArch64 Armv8-A cpu Cortex-A34.

In detail adding support for:
 - mcpu option in clang
 - AArch64 Target Features in clang
 - llvm AArch64 TargetParser definitions

details of the cpu can be found here:
https://developer.arm.com/ip-products/processors/cortex-a/cortex-a34

Reviewers: SjoerdMeijer

Reviewed By: SjoerdMeijer

Subscribers: SjoerdMeijer, kristof.beyls, hiraditya, cfe-commits,
llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D74483

Change-Id: Ida101fc544ca183a0a0e61a1277c8957855fde0b