scons: Move the build of ext/ into the variant dirs.

These are no longer split out and shared in the root build/ directory.
This does result in a small amount of overhead from building redundant
copies of these files, although the overhead is not significant. When
building 7 different variants of gem5, all the ISAs and NULL, the
difference on my machine was:

Before:
real    41m25.372s
user    914m22.266s
sys     41m51.816s

After:
real    42m38.074s
user    921m36.852s
sys     43m2.949s

This is about a 2-3% difference, which is a worse than typical case,
since the overhead scales with the number of variants being built.

The benefit of pulling ext/ into the variant directory is that there can
now be a single config which applies to all files used to build gem5,
and that config is represented by the variant of gem5 being built.

Change-Id: I6f0db97c63a7f3e252e7e351aa862340978e701b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56750
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>

ext/systemc/SConscript

@@ -26,8 +26,8 @@
 import os
 from m5.util.terminal import get_termcap
 
-Import('main')
-systemc = main.Clone()
+Import('env')
+systemc = env.Clone()
 
 build_root = Dir('.').abspath
 src_root = Dir('.').srcdir.abspath