ext: Revamp the systemc SConscripts.

The existing scripts were including pthread code and QT code at the same
time, and also insisting on an having a set of architecture specific
source files for whatever the current architecture is.

This change selects using either QT or pthreads based on the host
architecture, distributes accumulating source files, list source files
explicitly (to avoid including redundant coroutine libraries) and makes
scons insist on an architecture specific QT implementation only if QT is
being used. It also defines a preprocessor symbol which tells some headers
whether or not pthreads are being used, and also clones the scons
environment to avoid leaking flags into the main environment used to
compile gem5 itself.

If the host architecture isn't supported by systemc, a warning will be
printed, and the various build products and SConscript files will be
skipped over.

Change-Id: I1a40123a11e49e02922a054f093246cf197087bf
Reviewed-on: https://gem5-review.googlesource.com/3461
Reviewed-by: Matthias Jung <jungma@eit.uni-kl.de>
Reviewed-by: Christian Menard <christian.menard@tu-dresden.de>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>