[Executorch][Portable] Compile portable ops with -O2

Differential Revision: D64910578

Pull Request resolved: #6519

Author: kimishpatel

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

@@ -150,7 +150,7 @@ def define_op_library(name, deps, android_deps, aten_target, _allow_third_party_
             # library, and it blocks users like unit tests to use kernel
             # implementation directly. So we enable this for xplat only.
             ["-fvisibility=hidden"] if is_xplat() else []
-        ),
+        ) + get_compiler_optimization_flags(),
         deps = [
             "//executorch/runtime/kernel:kernel_includes" + aten_suffix,
         ] + deps,