Update on "Use c10 version of half/bfloat16 in executorch"

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.

Differential Revision: [D66106969](https://our.internmc.facebook.com/intern/diff/D66106969/)

[ghstack-poisoned]

Author: swolchok

runtime/core/portable_type/c10/targets.bzl

@@ -67,7 +67,7 @@ def define_common_targets():
             # fbsource//third-party/sleef:sleef currently fails to
             # link with missing symbols, hence the fbcode-specific dep below.
         }),
-        fbcode_exported_deps = [
+        fbcode_exported_deps = ([
             "//caffe2:aten-headers-cpu",
             "//caffe2:generated-config-header",
             "//caffe2/c10/core:base_headers",
@@ -76,7 +76,7 @@ def define_common_targets():
             "ovr_config//cpu:x86_64": [
                 "third-party//sleef:sleef",
             ]
-        }),
+        }) if not runtime.is_oss else [],
         fbcode_exported_preprocessor_flags = [
             # We don't -DCPU_CAPABILITY=AVX2 because that trips
             # -Wmacro-redefined, and we only care about getting