[vm/build] Expose is_release/is_profile so they are available in third_party gn scripts.

Follow-up to https://dart-review.googlesource.com/c/sdk/+/98434 as that broke hhh build(https://ci.chromium.org/p/dart/builders/ci.sandbox/flutter-engine-linux/1553).

Requires changes to flutter engine:
```
diff --git a/tools/gn b/tools/gn
index 7dacacf06..83bd7d9da 100755
--- a/tools/gn
+++ b/tools/gn
@@ -134,6 +134,9 @@ def to_gn_args(args):
     else:
         gn_args['dart_runtime_mode'] = runtime_mode

+    gn_args['is_release'] = gn_args['dart_runtime_mode'] == 'profile'
+    gn_args['is_product'] = gn_args['dart_runtime_mode'] == 'release'
+
     if args.dart_debug:
         gn_args['dart_debug'] = True

```

Change-Id: I2cf2473bade0f14cdd94fc9ff18febe313e26b55
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/98459
Reviewed-by: Siva Annamalai <asiva@google.com>