Put extension/parallel buck files back (#9232)

I shouldn't have deleted these, for the same reason I left
thread_parallel.h behind.

Author: swolchok

extension/parallel/TARGETS

@@ -0,0 +1,8 @@
+# Any targets that should be shared between fbcode and xplat must be defined in
+# targets.bzl. This file can contain fbcode-only targets.
+
+load(":targets.bzl", "define_common_targets")
+
+oncall("executorch")
+
+define_common_targets()

extension/parallel/targets.bzl

@@ -0,0 +1,22 @@
+load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "get_aten_mode_options", "runtime")
+
+def define_common_targets():
+    """Defines targets that should be shared between fbcode and xplat.
+
+    The directory containing this targets.bzl file should also contain both
+    TARGETS and BUCK files that call this function.
+    """
+
+    runtime.cxx_library(
+        name = "thread_parallel",
+        exported_headers = [
+            "thread_parallel.h",
+        ],
+        visibility = [
+            "//executorch/...",
+            "@EXECUTORCH_CLIENTS",
+        ],
+        deps = [
+            "//executorch/runtime/kernel:thread_parallel_interface",
+        ],
+    )