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

Summary:
X-link: pytorch/executorch#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: 265190624
exported-using-ghexport

Test Plan: builds

Reviewed By: shoumikhin

Differential Revision: D66106969

Author: swolchok

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(

c10/build.bzl

@@ -22,3 +22,15 @@ def define_targets(rules):
             [],
         ),
     )
+
+    rules.cc_library(
+        name = "c10_headers",
+        deps = [
+            "//c10/core:base_headers",
+            "//c10/macros",
+            "//c10/util:base_headers",
+            "//c10/util:bit_cast",
+            "//c10/util:ssize",
+        ],
+        visibility = ["//visibility:public"],
+    )

c10/core/build.bzl

@@ -90,6 +90,22 @@ def define_targets(rules):
         alwayslink = True,
     )
 
+    rules.cc_library(
+        name = "base_headers",
+        srcs = [],
+        hdrs = rules.glob(
+            [
+                "*.h",
+                "impl/*.h",
+            ],
+            exclude = [
+                "CPUAllocator.h",
+                "impl/alloc_cpu.h",
+            ],
+        ),
+        visibility = ["//visibility:public"],
+    )
+
     rules.filegroup(
         name = "headers",
         srcs = rules.glob(
@@ -101,5 +117,5 @@ def define_targets(rules):
                 "alignment.h",
             ],
         ),
-        visibility = ["//c10:__pkg__"],
+        visibility = ["//visibility:public"],
     )

c10/util/build.bzl

@@ -80,6 +80,18 @@ def define_targets(rules):
         ],
     )
 
+    rules.cc_library(
+        name = "base_headers",
+        hdrs = rules.glob(
+            ["*.h"],
+            exclude = [
+                "bit_cast.h",
+                "ssize.h",
+            ],
+        ),
+        visibility = ["//visibility:public"],
+    )
+
     rules.filegroup(
         name = "headers",
         srcs = rules.glob(