Add the switch logic for math conformance fast path

Add the switch logic for math conformance fast path

Modify the __ocl_math_fastpath_flag init value in the
backend link stage to switch between fast path and
conformance path.

V2:
    Rename the function prototype parameter name.

V3:
    Modify the parameter to boolean and correct some comment words.

Signed-off-by: Junyan He <junyan.he@linux.intel.com>
Reviewed-by: "Song, Ruiling" <ruiling.song@intel.com>