pw_toolchain: Fix setting the output directory for static_library

Pigweed's toolchain ignores target specified output directories of static
libraries. Fix this by properly using {{output_dir}} and setting
default_output_dir.

Change-Id: Iae4efb031c013d9b7660e07d17635595e17f3858
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/12820
Commit-Queue: Michael Spang <spang@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
diff --git a/pw_toolchain/generate_toolchain.gni b/pw_toolchain/generate_toolchain.gni
index 10dba7c..30e5b97 100644
--- a/pw_toolchain/generate_toolchain.gni
+++ b/pw_toolchain/generate_toolchain.gni
@@ -114,10 +114,10 @@
       tool("alink") {
         command = "rm -f {{output}} && ${invoker.ar} rcs {{output}} {{inputs}}"
         description = "ar {{target_output_name}}{{output_extension}}"
-        outputs = [
-          "{{target_out_dir}}/lib/{{target_output_name}}{{output_extension}}",
-        ]
+        outputs =
+            [ "{{output_dir}}/{{target_output_name}}{{output_extension}}" ]
         default_output_extension = ".a"
+        default_output_dir = "{{target_out_dir}}/lib"
       }
 
       lib_switch = "-l"