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

backends/xnnpack/CMakeLists.txt

@@ -129,7 +129,7 @@ if(NOT CMAKE_TOOLCHAIN_FILE MATCHES ".*(iOS|ios\.toolchain)\.cmake$")
   list(TRANSFORM _xnn_executor_runner__srcs PREPEND "${EXECUTORCH_ROOT}/")
   add_executable(xnn_executor_runner ${_xnn_executor_runner__srcs})
   target_link_libraries(
-    xnn_executor_runner xnnpack_backend gflags portable_ops_lib
+    xnn_executor_runner xnnpack_backend gflags portable_ops_lib executorch
   )
   target_compile_options(xnn_executor_runner PUBLIC ${_common_compile_options})
 endif()

runtime/kernel/test/CMakeLists.txt

@@ -47,14 +47,11 @@ add_executable(
 )
 target_link_libraries(
   operator_registry_max_kernel_num_test GTest::gtest GTest::gtest_main
-  GTest::gmock
+  GTest::gmock executorch
 )
 target_compile_definitions(
   operator_registry_max_kernel_num_test PRIVATE "-DMAX_KERNEL_NUM=1"
 )
-target_include_directories(
-  operator_registry_max_kernel_num_test PRIVATE ${EXECUTORCH_ROOT}/..
-)
 # TODO: This is currently not working!
 # add_test(operator_registry_max_kernel_num_test operator_registry_max_kernel_num_test)