scons,util: Make the tlm SConstruct put SConsign in the build dir.

Otherwise it ends up in the root of the source directory which makes it
harder to blast everything and do a truly clean build.

Change-Id: Id1e948b9059aaab0814c19c0cfb6ed611bf14d08
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42587
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

util/tlm/SConstruct

@@ -78,6 +78,7 @@ sys.path.append(gem5_root + '/src/python')
 AddOption('--no-colors', dest='use_colors', action='store_false',
           help="Don't add color to abbreviated scons output")
 
+main.SConsignFile('build/systemc/sconsign')
 SConscript(gem5_root + '/ext/systemc/SConscript',
            variant_dir='build/systemc',
            exports='main')