scons: Generate cxx_config/init.cc using a helper script.

Change-Id: Ib0129fc5c2de3977f2dedc1bfec532bbedb2d20e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49428
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
2 files changed