Implement char/short_store primitives at aligned addresses for ARM

* src/atomic_ops/sysdeps/gcc/arm.h: Include char_atomic_store.h and
short_atomic_store.h if AO_ARM_HAVE_LDREX (after
AO_ACCESS_x_CHECK_ALIGNED defined); add comment.
* src/atomic_ops/sysdeps/gcc/arm.h: Include
all_aligned_atomic_load_store.h instead of all_atomic_load_store.h
(only if not AO_ARM_HAVE_LDREX).