GBE: enable relocatable pch files.

GBE: enable relocatable pch files.

As by default, when include a pch file, clang need to make sure
the original header file is untouched. This is impossible when
we want to distribute a pch file to a new system. We need to
use the relocatable pch feature provided by clang here.
We now create two pch files. One is relocatable pch file which
is used to install to the system directory. The other is a local
pch file which is used during the build time. We need both pch
files because at the build time, we don't have an ocl_stdlib.h
in the system directory. The local pch file is used for the beignet's
build and the utest only. All the other applications will use
the installed pch/pcm files.

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>