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/fputils/SConscript

@@ -28,11 +28,11 @@
 #
 # Authors: Andreas Sandberg
 
-Import('main')
+Import('env')
 
-main.Prepend(CPPPATH=Dir('./include'))
+env.Prepend(CPPPATH=Dir('./include'))
 
-fpenv = main.Clone()
+fpenv = env.Clone()
 
 # By default gcc uses c89 and clang uses c99. For fputils to compile
 # we need to use c99.
@@ -44,6 +44,6 @@ fpenv.Library('fputils', [
         fpenv.SharedObject('fp80.c'),
     ])
 
-main.Append(LIBS=['fputils'])
-main.Prepend(LIBPATH=[Dir('.')])
+env.Append(LIBS=['fputils'])
+env.Prepend(LIBPATH=[Dir('.')])