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>