Add some missing files to sync2 for linux and darwin

core/sync/sync2/primitives_linux.odin

@@ -0,0 +1,15 @@
+//+build linux
+//+private
+package sync2
+
+// TODO(bill): remove libc
+foreign import libc "system:c"
+
+_current_thread_id :: proc "contextless" () -> int {
+	foreign libc {
+		syscall :: proc(number: i32, #c_vararg args: ..any) -> i32 ---
+	}
+
+	SYS_GETTID :: 186;
+	return int(syscall(SYS_GETTID));
+}