scons: Remove the "abstract" tag from Executable classes.

scons: Remove the "abstract" tag from Executable classes.

That tag was intended to mark an Executable subclass as abstract, aka
only suitable for using as bases for other Executable subclasses and not
for direct instantiation. The only place it was used was the base
Executable class however, and that class is actually directly useful
when setting up a generic executable from other gem5 sources.

Change-Id: I70204b63c03bb45bf21b8c312a7b8581be5e0cab
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40616
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>