arch: Convert exec keyword to exec() function

arch: Convert exec keyword to exec() function

The function syntax was also accepted in 2.x

Change-Id: I3d9bef3ddcd5e98ff589796d21dbfeb9796bff1d
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26252
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>