Clang: tune include paths used by clang

Do not search the standard system directories for include files, but do
search compiler builtin include directories. To make sure that the
include files for intrinsics are the ones clang can parse, put that
path first on the command-line.

Change-Id: I2ada992b58203d1c3dbd55851c5f195c12572943
Reviewed-by: Eike Ziller <eike.ziller@digia.com>