[PATCH] [ARM] ARMv8.6-a command-line + BFloat16 Asm Support

Summary:
This patch introduces command-line support for the Armv8.6-a architecture and assembly support for BFloat16. Details can be found
https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/arm-architecture-developments-armv8-6-a

in addition to the GCC patch for the 8..6-a CLI:
https://gcc.gnu.org/legacy-ml/gcc-patches/2019-11/msg02647.html

In detail this patch

- march options for armv8.6-a
- BFloat16 assembly

This is part of a patch series, starting with command-line and Bfloat16
assembly support. The subsequent patches will upstream intrinsics
support for BFloat16, followed by Matrix Multiplication and the
remaining Virtualization features of the armv8.6-a architecture.

Based on work by:
- labrinea
- MarkMurrayARM
- Luke Cheeseman
- Javed Asbar
- Mikhail Maltsev
- Luke Geeson

Reviewers: SjoerdMeijer, craig.topper, rjmccall, jfb, LukeGeeson

Reviewed By: SjoerdMeijer

Subscribers: stuij, kristof.beyls, hiraditya, dexonsmith, danielkiss, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D76062