scons: Add a mechanism to append flags when building particular files.

scons: Add a mechanism to append flags when building particular files.

This could be used to tweak settings for a particular file if it needed
special treatment. I plan to use this for protobuf files which generate
code that produce a warning in gcc 9 which turns into an error.

Change-Id: I53e2dac48cd68f0cc8ad9031d8484c5036d6e4a6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24923
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>