galileo: Revise include flags for LLVM Clang
galileo: Revise include flags for LLVM Clang

This patch revises the compiler flags when LLVM Clang is in use to
cause Clang to automatically use its built-in header files rather than
those for GCC.

MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,arurke/contiki,arurke/contiki,arurke/contiki