scons: Generate a single debug flag C++ file
scons: Generate a single debug flag C++ file

Reduces target count/compiler invocations by ~180.
(transplanted from 30b40ca619baf815aaa00a91007c0fc111f83837)

LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5