On 32-bit x86, 64-bit integers have 4-byte alignment, which resulted in
a non-integral type being selected for storage type in case of lock-based
atomic_ref. This broke arithmetic and bitwise operations on atomic_ref.
We now select an integral type based on its native alignment, not the storage
alignment we require for atomic operations. This is fine in case of lock-based
backend.
Also, extended buffer_storage with support for specifying alignment and removed
aligned_buffer_storage and storage128_t.
Lock-based operations have no reason to require object alignment higher
than alignof(T). This commit implements a special storage type for
lock-based operations, which has the same alignment as value_type.
Also, added tests to verify required_alignment correctness both
in lock-free and lock-based cases.
For some unknown reason, int128 bit operation tests started failing
in Travis CI for x86-64. The error does not reproduce locally. Given that
float128 were known to fail similarly (only on 32-bit target), it seems all
128-bit operations are broken in clang-5 (possibly caused by something
specific to Travis CI).
This commit re-enables 32-bit tests on clang-5, but disables int128 and
float128 tests on both 32 and 64-bit targets.
