tests,arch-arm: Pass gem5_root as an arg in run.py

tests,arch-arm: Pass gem5_root as an arg in run.py

Previously `tests/gem5/fs/linux/arm/run.py` contained an ugly,
hard-coded `gem5_root` variable. In this commit we pass `gem5_root`
as an argument from `tests/gem5/fs/linux/arm/test.py`, utilizing
`config.base_dir`.

Change-Id: I2b1e3369b1078cce9375fadb7c39fa4292648658
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33955
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>