Set kernel default visibility to hidden (#3060)

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.

---

This is not done in fbcode so far. When we compile in fbcode, seems that all dependency libraries is compiled into shared library, not static library. For example, op tests depends on op implementation through shared library. In that case, the hidden symbols are not exposed and could cause link time failure.

In xplat, these dependencies are set to static libraries so it has no impact. Only when we explicitly build a shared library (for android), we hide the symbols and rely on op registry to store the impl.

---

This applies to internal build only for now. We will re-visit this for OSS later. It's a step needed to make use of selective build for building shared library (android use case mainly)

Differential Revision: D56167833

Author: kirklandsign

shim/xplat/executorch/build/env_interface.bzl

@@ -216,7 +216,7 @@ env = struct(
     # @lint-ignore BUCKLINT: native and fb_native are explicitly forbidden in fbcode.
     genrule = native.genrule,
     is_oss = True,
-    is_xplat = False,
+    is_xplat = lambda: False,
     patch_deps = _patch_deps,
     patch_cxx_compiler_flags = _patch_cxx_compiler_flags,
     patch_executorch_genrule_cmd = _patch_executorch_genrule_cmd,

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

@@ -1,4 +1,4 @@
-load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
+load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "is_xplat", "runtime")
 load("@fbsource//xplat/executorch/build:selects.bzl", "selects")
 
 def op_target(name, deps = [], android_deps = [], _allow_third_party_deps = False, _aten_mode_deps = []):
@@ -122,7 +122,9 @@ 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"] if is_xplat() else []
+        ),
         deps = [
             "//executorch/runtime/kernel:kernel_includes" + aten_suffix,
         ] + deps,