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 <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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