scons: Remove explicitly set defaults in calls to AddOption.

scons: Remove explicitly set defaults in calls to AddOption.

Like the optparse module's add_option method, most keyword arguments
have sensible default values. We should avoid setting those explicitly
when calling AddOption, since it usually doesn't make anything clearer
and just adds more text to wade through.

Change-Id: I70e425d9f1a0da1cafcc3d7dd24bfde18c0b3f35
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32116
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>