GBE: fix bugs when handling -cl-std option.

GBE: fix bugs when handling -cl-std option.

Actually, CLANG does take this option and we should not
filter it out. We also change the default option to create
PCH file to -cl-std=CL1.2. And if the user pass in a CL1.1
we will have to disable PCH.

Another change is that if we are CL1.2, then we should enable
the cl_khr_fp64 by default. As from CL1.2, this extension should
be enabled by default.

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>
Reviewed-by: "Yang, Rong R" <rong.r.yang@intel.com>