Set ldflags at top of modular config

This avoids the possiblity to overwrite ldflags, as that is
explicitly done with a variable being set to the empty list.

b/302733082

starboard/build/config/modular/BUILD.gn

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 config("modular") {
+  ldflags = []
   cflags = [
     "-ffunction-sections",
     "-fdata-sections",
@@ -171,8 +172,6 @@ config("modular") {
     defines += [ "THREAD_SANITIZER" ]
   }
 
-  ldflags = []
-
   # Wrap underlying POSIX implementations to be cross-compatible with
   # musl-based types that are used in modular builds. Evergreen builds use
   # a different map-based wrapping of these POSIX APIs.