[dart/fuzzer] Restrict libFuzzer to linux

Rationale:
Mac and possibly Windows build don't love fuzzing yet.
Change-Id: Ided9734285197e51145102532e1f59550e261bcf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/101342
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Aart Bik <ajcbik@google.com>
diff --git a/runtime/bin/BUILD.gn b/runtime/bin/BUILD.gn
index 2c1a14d..532229e 100644
--- a/runtime/bin/BUILD.gn
+++ b/runtime/bin/BUILD.gn
@@ -1090,7 +1090,7 @@
 }
 
 # DartLibFuzzer only "exists" for restricted configurations.
-if (defined(is_asan) && is_asan &&
+if (defined(is_linux) && is_linux && defined(is_asan) && is_asan &&
     (dart_target_arch == "x64" || dart_target_arch == "arm64")) {
   dart_executable("dart_libfuzzer") {
     extra_ldflags = [ "-fsanitize=address,fuzzer" ]