scons: Enable explicitly building something with no variant.

scons: Enable explicitly building something with no variant.

The SConstruct currently expects all explicit targets to be under a
variant directory like ARM or X86 which tells it what settings to use,
etc. There are things which scons knows how to build however, which do not
live under a variant directory, specifically everything under ext.

This change makes scons not look for a variant directory when it
encounters a target which is built by something in ext. This enables
things like explicitly building the systemc libraries, for example.

Change-Id: I8982a96fe49e3cb970ec78e11cea08703990c686
Reviewed-on: https://gem5-review.googlesource.com/3460
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Christian Menard <christian.menard@tu-dresden.de>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>