Metal: Move ldflags to Metal backend config. ldflags apparently don't work in source_sets. Bug: chromium:1014228 Change-Id: I123900ae7f7ef3705301dc9b78cdf92ebd0e5c4f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1873235 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>