[Executorch][Portable] Compile portable ops with -O2 (#6588)

Pull Request resolved: #6519

prefill toks: 21
decode: 11
after: e2e 621ms, prefil:: 370ms, decode: 43.84 tok/sec
ghstack-source-id: 251021625
@exported-using-ghexport

Differential Revision: [D64910578](https://our.internmc.facebook.com/intern/diff/D64910578/)

---------

Co-authored-by: Kimish Patel <[email protected]>

Author: pytorchbot

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,