libatomic: Use config/mingw/lock.c for --enable-threads=single

Without this change bootstrap fails for x86_64-w64-mingw32 with
--disable-threads=single because there is no lock.c file chosen by
libatomic's configure.

libatomic/ChangeLog:

	* configure.tgt (config_path) [target_thread_file=single]:
	Use 'mingw' config.