root,roofit: lower debug information from -g3 to -g

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
root,roofit: use --buid=debug and LLVMDEV=1

The following should provide ROOT6 build with debug information
(incl. LLVM/Clang stack)

Drop direct debug flags from {C,CXX}FLAGS.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>