pw_build: Fix linker script defines

For some reason this world may never know, I set up pw_linker_script
using process_file_template to iterate over defines. While this works
for most defines, it breaks when a define contains a file path. Switches
to use a foreach instead as it contains less internal magic.

Change-Id: I974a63fc6d769fa884b9c867944112d068453599
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/21500
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Armando Montanez <amontanez@google.com>
diff --git a/pw_build/linker_script.gni b/pw_build/linker_script.gni
index 8c7ec55..7ba64ac 100644
--- a/pw_build/linker_script.gni
+++ b/pw_build/linker_script.gni
@@ -83,7 +83,9 @@
 
     # Add defines.
     if (defined(invoker.defines)) {
-      args += process_file_template(invoker.defines, "-D{{source_name_part}}")
+      foreach(compiler_define, invoker.defines) {
+        args += [ "-D${compiler_define}" ]
+      }
     }
 
     # Set output file.