Use clang as preprocessor when clang toolchain is selected
Use clang as preprocessor when clang toolchain is selected

Change-Id: I562c5de91e12fe384245df41225dfb9122a13a85
Signed-off-by: Roberto Vargas <d75e640c49defb97bd074f828210df57ed181e1f@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware