GBE: fixed the stack allocation.

GBE: fixed the stack allocation.

Yongjia wrote a case hit the previous 1KB limitation. I took a look at
the stack pointer related code then I found the implementation is not
comply with the OCL spec.

According to OpenCL spec, section 6.9:

d. Variable length arrays and structures with flexible (or unsized) arrays are not supported.

Thus all the local variable size should be constant, and we can
manipulate the stack pointer easier , no need to do the alignment
calculating at runtime, and could get the eaxct stack size then
allocate stack size on demand. I still put a limitation there which
is 64KB.

v2:
don't add the step if the step is zero.

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