x86,scons: De-indent the main x86 SConscript file.

x86,scons: De-indent the main x86 SConscript file.

Rather than put all the declaration of sources in the body of an "if", if
the "if" wouldn't happen, exit from the SConscript entirely. Then the
other parts of the SConscript can be totally unindented. Also wrap some
lines which were longer than 80 characters.

Change-Id: I113d649cdd051da02d5ab14a4547b26113d2f7ef
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34161
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>