devel/gprbuild: Improve option DEBUG

devel/gprbuild:

• Currently options DEBUG is ${BROKEN} because of some warnings from libgpr and between the Port and the GNAT tool-chain. Replace the ${BROKEN} statement with a new variable that is passed to the libgrp do-build phase that attenuates the tool-chain warnings
  
  PR: 296655

(cherry picked from commit b665e89e9a64a1bbf2250dbfc779e69627ecf1f3)