[OpenCL] Set half-precision Div and Sqrt accuracy (#179621)
OpenCL spec relaxed half-precision divide to 1 ULP and sqrt to 1.5 ULP
in https://github.com/KhronosGroup/OpenCL-Docs/pull/1293
https://github.com/KhronosGroup/OpenCL-Docs/pull/1386
This can enable target to use hardware rcp instruction for half.