[ARM] Armv8.6-a Matrix Mul cmd line support

This patch upstreams support for the Armv8.6-a Matrix Multiplication
Extension. A summary of the features can be found here:

https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/arm-architecture-developments-armv8-6-a

This patch includes:

- Command line options to enable these features with +i8mm, +f32mm, or f64mm

Note: +f32mm and +f64mm are optional and so are not enabled by default

This is part of a patch series, starting with BFloat16 support and
the other components in the armv8.6a extension (in previous patches
linked in phabricator)

Based on work by:
- Luke Geeson
- Oliver Stannard
- Luke Cheeseman

Reviewers: t.p.northover, DavidSpickett

Reviewed By: DavidSpickett

Subscribers: DavidSpickett, ostannard, kristof.beyls, danielkiss,
cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77875