Use c10 version of half/bfloat16 in executorch (#144111)

Summary:
X-link: https://github.com/pytorch/executorch/pull/7040

Accomplished by importing relevant files from c10 into
executorch/runtime/core/portable_type/c10, and then using `using` in
the top-level ExecuTorch headers. This approach should keep the
ExecuTorch build hermetic for embedded use cases. In the future, we
should add a CI job to ensure the c10 files stay identical to the
PyTorch ones.
ghstack-source-id: 260047850
exported-using-ghexport

Test Plan: builds

Differential Revision: D66106969

Pull Request resolved: https://github.com/pytorch/pytorch/pull/144111
Approved by: https://github.com/malfet

buckbuild.bzl

@@ -997,6 +997,7 @@ def define_buck_targets(
             "Config.h": ":generate_aten_config[Config.h]",
         },
         labels = labels,
+        visibility = ["PUBLIC"],
     )
 
     fb_xplat_cxx_library(