pw_rust: Enable GN pw_rust targets for linux host

- Having GN rust build checked in CQ.
- Fix a issue in bindgen

Change-Id: I6c313dd95c3c83028f3555be2da075feb0428a3f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/254372
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Jiacheng Lu <lujc@google.com>
Reviewed-by: Erik Gilling <konkers@google.com>
Docs-Not-Needed: Jiacheng Lu <lujc@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>

pw_build/rust_bindgen_action.gni

@@ -39,7 +39,7 @@ template("rust_bindgen_action") {
     program = _bindgen_exe_path
     header = rebase_path(invoker.header, root_build_dir)
 
-    if (invoker.libclang_path) {
+    if (defined(invoker.libclang_path)) {
       # Note `-isysroot` may not work properly when `LIBCLANG_PATH` is set.
       env = [ "LIBCLANG_PATH=" + invoker.libclang_path ]
     }

targets/host/target_toolchains.gni

@@ -86,6 +86,7 @@ _linux_config = {
   pw_bloat_BLOATY_CONFIG = get_path_info("linux.bloaty", "abspath")
   pw_unit_test_AUTOMATIC_RUNNER = get_path_info("run_test", "abspath")
   pw_async2_DISPATCHER_BACKEND = "$dir_pw_async2_epoll:dispatcher_backend"
+  pw_rust_ENABLE_EXPERIMENTAL_BUILD = true
 }
 
 # macOS-specific target configuration.