system-arm: gitignore the aarch64 bootloader object files

Change-Id: I0cb494fb82c557cfce553bc925ea89a8220e4c16
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22826
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed