base: Fix undefined behavior in mask generation

base: Fix undefined behavior in mask generation

When generating a mask, if the number of bits is greater than
the maximum shift distance (63), the shift will have undefined
behavior. Previously the branch was taking care of a single
trespassing case, and it has been fixed to cover the remaining.

Issue-on: https://gem5.atlassian.net/browse/GEM5-205

Change-Id: Ib5a00917c8d2b23ffdb710c2f9673d956cd9f43e
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27104
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>