[dart2wasm] Add extra build targets in V8 build documentation

Change-Id: Ibef9a043a1326651b123bb0a3748407c6cc87b18
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/366941
Reviewed-by: Ömer Ağacan <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: mkustermann

pkg/dart2wasm/docs/v8.md

@@ -9,7 +9,7 @@ Assuming `depot_tools` is installed and in `PATH`:
 ```
 % mkdir v8-gclient && cd v8-gclient
 v8-gclient % fetch v8 && cd v8
-v8-gclient/v8 % tools/dev/gm.py x64.release
+v8-gclient/v8 % tools/dev/gm.py x64.release d8 wami
 ...
 ```
 
@@ -26,7 +26,7 @@ v8-gclient/v8 % vim out/x64.release/args.gn
 ...
 enable_profiling = true
 ...
-v8-gclient/v8 % tools/dev/gm.py x64.release
+v8-gclient/v8 % tools/dev/gm.py x64.release d8 wami
 ...
 ```