scons: Put internal build files in a gem5.build directory.

This keeps them organized, and also creates an anchor for the build
directory other than a directory named "build".

Change-Id: I3ed2f569e9fcd62eb4eca1c4556b45cd0c3552d4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56589
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
2 files changed