tests: Fixed .testignore from 'arch64' to 'aarch64'

tests: Fixed .testignore from 'arch64' to 'aarch64'

Entries in `tests/gem5/.testignore` which were ignoring insttest tests,
were stating the host system as 'arch64' instead of 'aarch64'. This has
been fixed.

Change-Id: Ib90bd89e0544d225afc012fefca98db0ea2d8dd0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25845
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>
Maintainer: Jason Lowe-Power <jason@lowepower.com>