Implement workaround for IBM405 Errata 77 (aka CPU_210), where
interrupted stwcx. may errantly write data to memory:

https://elinux.org/images/1/1d/Ppc405gp-errata.pdf

This is because stwcx. is split into two pieces in the pipeline.

We need to
(1) insert dcbt before every stwcx. instruction, as well as
(2) insert sync before every rfi/rfci instruction.

It is unclear which processors are affected, but according to Linux,
all 405-based cores up until 405GPR and 405EP are affected:

https://github.com/torvalds/linux/blob/master/arch/powerpc/platforms/40x/Kconfig#L140

For kernel, this workaround can be restricted to affected processors.
However, for kernel modules and userland, we have to enable it for all
32bit powerpc archs in order to share common binaries as before.

Proposed on port-powerpc:

http://mail-index.netbsd.org/port-powerpc/2020/02/21/msg003583.html

branches: 1.2.56; 1.2.60;
Pull in the Atheros HAL options so that OPENBLOCKS266_OPT will build.

branches: 1.1.2; 1.1.4; 1.1.6; 1.1.8;
Support Plathome's OpenBlockS600.