Implement -print-file-name=include (which is undocumented
but used by some Linux boot loaders). This option prints
out the directory holding the include files needed by
a freestanding program. The default implementation of
this doesn't work on FreeBSD because of the different
include file layout. But it's easy to implement:
just return /usr/include (or the cross-compiling equivalent).

Reviewed by: kan
MFC after: 1 week