scons: Define the rules for building debug flag hdrs in place.
scons: Define the rules for building debug flag hdrs in place.

Define the rules for building debug flag header files in place, instead
of looping over them all after they've been accumulated.

Change-Id: I02113a21529958c3f971b5462ea340d70d1b18d7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48372
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Hoa Nguyen <a0aa8449aacb303edcb25d6bbe69da9124f010fc@ucdavis.edu>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5