[build] Build libcxx ourselves for Mac ASAN too.

Bug: #60720
Change-Id: I5158610702dd2d3da7146187dc895f8303eb92be
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/428400
Commit-Queue: Daco Harkes <[email protected]>
Reviewed-by: Daco Harkes <[email protected]>

Author: rmacnak-google

build/config/BUILDCONFIG.gn

@@ -256,7 +256,8 @@ if (current_os == "win") {
 # BUILD OPTIONS
 # =============================================================================
 
-use_flutter_cxx = is_clang && (is_msan || is_tsan || is_ios)
+use_flutter_cxx =
+    is_clang && ((is_asan && is_mac) || is_msan || is_tsan || is_ios)
 
 # =============================================================================
 # TARGET DEFAULTS