Mac ld doesn't support --allow-multiple-definition

Change-Id: Ic5c3680609fa0b7454dedabfa82cec1999da0e61
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1123416
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 84b46b4..5912e7e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1507,7 +1507,9 @@
     # Allow multiple definitions because Bazel's rust_static_library includes
     # transitive C++ dependencies, which can lead to duplicate symbols when
     # linking multiple Rust libraries together.
-    ldflags = [ "-Wl,--allow-multiple-definition" ]
+    if (!is_mac) {
+      ldflags = [ "-Wl,--allow-multiple-definition" ]
+    }
   }
 }