Fix register fetch/store order for native AArch64 Linux

I noticed we don't handle register reads/writes in the best way for native
AArch64 Linux.  Some other registers are fetched/stored even if upper level
code told us to fetch a particular register number.

Fix this by being more strict about which registers we touch when
reading/writing them in the native AArch64 Linux layer.

There should be no user-visible changes due to this patch.

Regression-tested on aarch64-linux Ubuntu 22.04/20.04.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>