[build] Put less into gen dir to avoid copying to test shards.

obj is filtered from the testing shard file sets but gen is not, and some tests depend on things in gen.

TEST=ci
Change-Id: I19bc763935c1cfd5077b2aeaf83e37f992ab785c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508281
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

.gn

@@ -19,4 +19,7 @@ script_executable = "python3"
 default_args = {
   # Overwrite default args declared in the Fuchsia sdk
   fuchsia_sdk_readelf_exec = "//buildtools/linux-x64/clang/bin/llvm-readelf"
+
+  # Dart uses of a copy of icudata embedded into the executable
+  icu_copy_icudata_to_root_build_dir = false
 }