Metal: Align OS macro usage Consistenty used TARGET_OS_* macro instead of ANGLE_PLATFORM_* in the Metal backend code for better searchability. Bug: angleproject:360147119 Change-Id: I754ddc158daa6b1894b0b4fb0489d59f14605b66 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5816828 Reviewed-by: Kimmo Kinnunen <kkinnunen@apple.com> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>