libcxx-test-fix-lockfree-test-for-i386

libcxx-test-fix-lockfree-test-for-i386

Lock is_always_lock free test fails on i386 because std::atomic is aligned
to 8 bytes while long long is aligned to 4 bytes. clang can't generate inline
code for unaligned 8 byte atomics even tough instruction set and gcc support
it.

That makes it expected thaqt ATOMIC_LLONG_LOCK_FREE and
std::atomic<long long>::is_always_lock_free don't match on i386. Correct test
for std::atomic<long long> is to check if target cpu support cmpxchg8 instruction.
To set instruction support one can check __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 define.

Bug: https://llvm.org/bugs/show_bug.cgi?id=19355

Gbp-Pq: Topic libcxx
Gbp-Pq: Name libcxx-test-fix-lockfree-test-for-i386.patch