Metal: Gate mathMode usage on compile time SDK versions. Integrators of ANGLE who compile with SDK versions lower than macOS 15 would get compile errors by referencing mathMode. Bug: chromium:383994655 Change-Id: I152403977c1233e4049f13f3d99cdd12fdf87073 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6973058 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Avi Drissman <avi@chromium.org>