[0/3] [2.42 Backport] aarch64: Remove B16B16, SVE2p1 and SME2p1

[Andrew Carlotti]

Unfortunately the support for these extensions in the 2.42 release has a large
number of missing instructions and wrong code bugs.  I expect it will still
take a long time to fix all of these issues and be confident that we haven't
missed any more problems.  This level of new feature support is not something
we would normally consider backporting, and I don't think it's appropriate to
do so in this case either.

Instead, this patch series disables support for these extensions in the 2.42
branch.  It will then be possible to make a point release with the broken
behaviour removed, without waiting for all of the bugs and omissions in these
extensions to be fixed on master.  It also avoids the burden of having to
manage and verify a large number of bugfix patches on two different branches.

Is this ok for the 2.42 branch?