watchOS target os

- add support for --os=watchos | --os=watchos_simulator
- do not depend on perfetto if dart_support_perfetto is false
- change the default value for dart_support_perfetto to !is_watchos

To minimize changes and to avoid modifying third-party zlib BUILD file, is_watchos implies is_ios for now.

TEST=manual, no watchOS SDK on bots yet.

Cq-Include-Trybots: luci.dart.try:vm-mac-debug-arm64-try,vm-mac-release-arm64-try
Change-Id: If2130068ef546162a07a9ba53f94b11ff25fb565
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/415021
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Ivan Inozemtsev <iinozemtsev@google.com>

utils/aot_snapshot.gni

@@ -174,7 +174,7 @@ template("aot_snapshot") {
     output_prefix = "lib"
     output_extension = ""
 
-    if (current_os == "mac" || current_os == "ios") {
+    if (current_os == "mac" || current_os == "ios" || current_os == "watchos") {
       output_extension = "dylib"
     } else if (current_os == "win") {
       output_extension = "dll"