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

shim/xplat/executorch/build/env_interface.bzl

@@ -119,7 +119,7 @@ def _remove_platform_specific_args(kwargs):
     keys = []
     for key in kwargs:
         if (key.endswith("_platform_preprocessor_flags") or key.endswith("_platform_deps") or
-            key.startswith("fbobjc") or key.endswith("_platform_compiler_flags")):
+            key.startswith("fbobjc") or key.endswith("_platform_compiler_flags") or key == "fbcode_exported_preprocessor_flags"):
             keys.append(key)
     for key in keys:
         kwargs.pop(key)