Fix missing output folder on making auto-generated headers (Automake)

Fix missing output folder on making auto-generated headers (Automake)

Create the directories before sed needs them (otherwise build fails
when sed tries to redirect output to a non-existent directory in case
of build root folder is not identical to source root folder).

* src/Makefile.am (atomic_ops/generalize-small.h,
atomic_ops/generalize-arithm.h, atomic_ops/sysdeps/ao_t_is_int.h,
atomic_ops/sysdeps/gcc/generic-arithm.h,
atomic_ops/sysdeps/loadstore/*.h): Add mkdir for output folder.