Move all os specific stuff for the runtime to one file

core/runtime/os_specific.odin

@@ -0,0 +1,45 @@
+package runtime
+
+when ODIN_OS == "freestanding" {
+	_OS_Errno  :: distinct int;
+	_OS_Handle :: distinct uintptr;
+
+	os_stdout :: proc "contextless" () -> _OS_Handle {
+		return 1;
+	}
+	os_stderr :: proc "contextless" () -> _OS_Handle {
+		return 2;
+	}
+
+	// TODO(bill): reimplement `os.write`
+	os_write :: proc(fd: _OS_Handle, data: []byte) -> (int, _OS_Errno) {
+		return 0, -1;
+	}
+
+	current_thread_id :: proc "contextless" () -> int {
+		return 0;
+	}
+
+} else {
+	import "core:os"
+
+	_OS_Errno  :: distinct int;
+	_OS_Handle :: os.Handle;
+
+	os_stdout :: proc "contextless" () -> _OS_Handle {
+		return os.stdout;
+	}
+	os_stderr :: proc "contextless" () -> _OS_Handle {
+		return os.stderr;
+	}
+
+	// TODO(bill): reimplement `os.write`
+	os_write :: proc(fd: _OS_Handle, data: []byte) -> (int, _OS_Errno) {
+		n, err := os.write(fd, data);
+		return int(n), _OS_Errno(err);
+	}
+
+	current_thread_id :: proc "contextless" () -> int {
+		return os.current_thread_id();
+	}
+}