Set kernel default visibility to hidden

Summary:
When we compile the kernel into a shared library, we don't know whether the definition of kernel implementation symbol can be dropped or not based on op registry. The kernel itself is just a normal function and the user can find it. We set its visibility to hidden by default. Then these kernels are gone when we do `objdump -TC`

This reduces binary size.

Differential Revision: D56167833

Author: kirklandsign

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

@@ -122,7 +122,7 @@ def define_op_library(name, deps, android_deps, aten_target, _allow_third_party_
         fbandroid_platform_deps = android_deps,
         # kernels often have helpers with no prototypes just disabling the warning here as the headers
         # are codegend and linked in later
-        compiler_flags = ["-Wno-missing-prototypes"],
+        compiler_flags = ["-Wno-missing-prototypes", "-fvisibility=hidden"],
         deps = [
             "//executorch/runtime/kernel:kernel_includes" + aten_suffix,
         ] + deps,