internal/cocoainit: move app/internal/cocoainit

We're about to move app/headless to gpu/headless, and it imports
cocoainit.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

internal/cocoainit/cocoa_darwin.go

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// Package cocoainit initializes support for multithreaded
+// programs in Cocoa.
+package cocoainit
+
+/*
+#cgo CFLAGS: -xobjective-c -fmodules -fobjc-arc
+#import <Foundation/Foundation.h>
+
+static inline void activate_cocoa_multithreading() {
+    [[NSThread new] start];
+}
+#pragma GCC visibility push(hidden)
+*/
+import "C"
+
+func init() {
+	C.activate_cocoa_multithreading()
+}