Add generic implementation of AO_double_load primitives

Add generic implementation of AO_double_load primitives

* src/Makefile.am (nobase_private_HEADERS): Add
double_atomic_load_store.h entry.
* src/Makefile.am (atomic_ops/generalize-small.h):
Generate primitives for AO_double_t as well (from the template).
* src/atomic_ops/generalize-small.h: Regenerate.
* src/atomic_ops/sysdeps/generic_pthread.h (AO_double_load_full): New
primitive.
* src/atomic_ops/sysdeps/loadstore/double_atomic_load_store.h: New
header (most code copied from atomic_load_store template).
* tests/test_atomic_include.template (test_atomicXX): Add test of
AO_double_loadXX.