As all Allwinner platforms are single-cluster A53 chips, we can disable
support for newer, unsupported architecture extensions. We can also
avoid some cache maintenance code, since no platform-specific setup is
required to enable coherency.

These changes reduce the size of .text on a default build with GCC 9.1
enough that .vectors again fits in the second half of a page, instead
of requiring its own page.

This commit was boot-tested on the Pinebook.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Change-Id: Ib90454ef0c798d5e714b7780c585be0b1ed49c6d