Fix test_atomic failure caused unaligned AO_double_t access on x86

Fix test_atomic failure caused unaligned AO_double_t access on x86

The failure is caused by violation of an assertion that checks
AO_double_t variable is 8-byte aligned on x86.

* doc/README.txt (AO_double_t): Add note about required alignment.
* src/atomic_ops/sysdeps/standard_ao_double_t.h (AO_double_t): Add
comment about alignment.
* src/atomic_ops_stack.h (AO_stack_t): Likewise.
* tests/test_atomic_include.template (test_atomicXX): Define old_w, w
local variables as static (as otherwise, e.g., they could have 4-byte
alignment on x86); add comment.