Relocate rust_linkable_symbol from fbcode_macros to fbsource

Summary:
Just like the previous migration {D81048274} affecting `rust_library` and `rust_binary` and `rust_unittest`, this time for `rust_linkable_symbol`.

It used to make sense to have `rust_linkable_symbol` inside fbcode_macros's build_defs because it used fbcode_macros's rust_library.

Now that fbcode_macros no longer has its own `rust_library` and there is one unified `rust_library` for fbsource in fbsource's build_defs, `rust_linkable_symbol` should be there too.

Reviewed By: JakobDegen

Differential Revision: D81541808

fbshipit-source-id: c68a8aabecfed543d834febca36d68fee6618497

Author: David Tolnay

shim/build_defs/rust_linkable_symbol.bzl

@@ -6,14 +6,6 @@
 # of this source tree. You may select, at your option, one of the
 # above-listed licenses.
 
-load("@prelude//rust:linkable_symbol.bzl", prelude_rust_linkable_symbol = "rust_linkable_symbol")
-load("@shim//:shims.bzl", _rust_library = "rust_library")
+load("@fbsource//tools/build_defs:rust_linkable_symbol.bzl", _rust_linkable_symbol = "rust_linkable_symbol")
 
-def rust_linkable_symbol(
-        visibility = ["PUBLIC"],
-        **kwargs):
-    prelude_rust_linkable_symbol(
-        visibility = visibility,
-        rust_library_macro = _rust_library,
-        **kwargs
-    )
+rust_linkable_symbol = _rust_linkable_symbol

shim/tools/build_defs/rust_linkable_symbol.bzl

@@ -0,0 +1,19 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is dual-licensed under either the MIT license found in the
+# LICENSE-MIT file in the root directory of this source tree or the Apache
+# License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+# of this source tree. You may select, at your option, one of the
+# above-listed licenses.
+
+load("@prelude//rust:linkable_symbol.bzl", prelude_rust_linkable_symbol = "rust_linkable_symbol")
+load("@shim//:shims.bzl", _rust_library = "rust_library")
+
+def rust_linkable_symbol(
+        visibility = ["PUBLIC"],
+        **kwargs):
+    prelude_rust_linkable_symbol(
+        visibility = visibility,
+        rust_library_macro = _rust_library,
+        **kwargs
+    )