Add a default system include of '/include'. This isn't particularly

useful when using Clang as a system-compiler, but its harmless. When
using Clang as a cross-compiler, this can be very handy as quite a few
toolchains ship their libc headers here rather than under
'/usr/include'.

For reference, this is the beginning of my work to also make the Clang
driver more suitable as a cross-compiler.

llvm-svn: 144089