arch-arm,arch-x86: Added missing overrides

arch-arm,arch-x86: Added missing overrides

These overrides were missing, causing compilation errors when
compiling with clang 9. Noted here:
https://www.mail-archive.com/gem5-dev@gem5.org/msg35657.html

Change-Id: I6d09a0e57af3131b9172d01468d2cdcf4b444c5d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33817
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>