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.
Update AUTHORS file
Fix missing output folder on making auto-generated test files (Automake)

* tests/Makefile.am (test_atomic_include.h, list_atomic.c,
list_atomic.i): Add mkdir for output folder.
Relax shareability domain for dmb st in AO_nop_write (ARM/AArch64)

* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_nop_write): Replace "dmb st"
to "dmb ishst" (this way the barriers will occupy the inner shareable
domain).
* src/atomic_ops/sysdeps/gcc/arm.h (AO_nop_write): Likewise.
Update AUTHORS file