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/libelf/SConscript

@@ -30,7 +30,7 @@
 
 import os, subprocess
 
-Import('main')
+Import('env')
 
 elf_files = []
 def ElfFile(filename):
@@ -97,7 +97,7 @@ ElfFile('libelf_convert.c')
 ElfFile('libelf_fsize.c')
 ElfFile('libelf_msize.c')
 
-m4env = main.Clone()
+m4env = env.Clone()
 if m4env['GCC']:
     m4env.Append(CCFLAGS=['-Wno-pointer-sign',
                           '-Wno-unused-but-set-variable',
@@ -146,6 +146,6 @@ m4env.Library('elf', [m4env.SharedObject(f) for f in elf_files])
 m4env.Command(File('native-elf-format.h'), File('native-elf-format'),
               '${SOURCE} > ${TARGET}')
 
-main.Prepend(CPPPATH=Dir('.'))
-main.Append(LIBS=[File('libelf.a')])
-main.Prepend(LIBPATH=[Dir('.')])
+env.Prepend(CPPPATH=Dir('.'))
+env.Append(LIBS=[File('libelf.a')])
+env.Prepend(LIBPATH=[Dir('.')])