Update on "[Executorch][optimized] Fix op_div impl to use portable for fallback path"

Earlier we just copy pasted from portable impl. This diff refactors portable to
make it usable from optimized lib. As a result we get all the size reduction
benefit from build and size optimizations landed in portable.

Differential Revision: [D65606665](https://our.internmc.facebook.com/intern/diff/D65606665/)

[ghstack-poisoned]

Author: kimishpatel

shim/xplat/executorch/kernels/portable/op_registration_util.bzl

@@ -1271,4 +1271,4 @@ def portable_source_list():
 
 def portable_header_list():
     """All the header file names from //executorch/kernels/portable/cpu/"""
-    return ["selective_build.h", "scalar_utils.h", "math_constants.h", "vec_ops.h"]
+    return ["selective_build.h", "scalar_utils.h", "math_constants.h", "vec_ops.h", "op_div_impl.h"]