Add internal header containing only char/short/int/AO_t atomic loads
(code refactoring)

* src/Makefile.am (nobase_private_HEADERS): Add all_atomic_only_load.h
entry.
* src/atomic_ops/sysdeps/all_aligned_atomic_load_store.h: Include
all_atomic_load_store.h instead of including X_atomic_load.h and
X_atomic_store.h directly; add comment for char type.
* src/atomic_ops/sysdeps/all_atomic_load_store.h: Include
all_atomic_only_load.h instead of including X_atomic_load.h directly.
* src/atomic_ops/sysdeps/all_atomic_only_load.h: New file (contains
only inclusion of atomic_load.h, char_atomic_load.h,
short_atomic_load.h and int_atomic_load.h files.