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 <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-by: Christian Menard <d37c2c10add41f490d0e6937549a71a42b7410e1@tu-dresden.de>
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>

gem5/gem5,gem5/gem5,rallylee/gem5,powerjg/gem5-ci-test,powerjg/gem5-ci-test,powerjg/gem5-ci-test,rallylee/gem5,rallylee/gem5,gem5/gem5,rallylee/gem5,TUD-OS/gem5-dtu,sobercoder/gem5,powerjg/gem5-ci-test,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,powerjg/gem5-ci-test,sobercoder/gem5,sobercoder/gem5,rallylee/gem5,rallylee/gem5,rallylee/gem5,gem5/gem5,gem5/gem5,powerjg/gem5-ci-test,TUD-OS/gem5-dtu,sobercoder/gem5,sobercoder/gem5,sobercoder/gem5,gem5/gem5,TUD-OS/gem5-dtu,sobercoder/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,powerjg/gem5-ci-test,gem5/gem5