build: silence duplicate -lobjc linker warning on Xcode 15+

Add -Wl,-no_warn_duplicate_libraries to the darwin cgo LDFLAGS and allow
it through the cgo flag allowlist via CGO_LDFLAGS_ALLOW in the build
scripts, so the ld-prime duplicate -lobjc note no longer clutters build
output.

activation_policy_darwin.go

@@ -5,6 +5,12 @@ package main
 /*
 #cgo CFLAGS: -x objective-c -fobjc-arc
 #cgo LDFLAGS: -framework AppKit
+// Silence the "ignoring duplicate libraries: '-lobjc'" note emitted by the
+// Xcode 15+ linker (ld-prime): cgo links -lobjc for our Objective-C code and
+// the Go darwin runtime pulls it in as well, which the new linker flags as a
+// duplicate. Suppressing it here keeps the build output clean; the flag is a
+// no-op on the legacy ld64 path.
+#cgo LDFLAGS: -Wl,-no_warn_duplicate_libraries
 #include <dispatch/dispatch.h>
 #import <AppKit/AppKit.h>