[patch] libstdc++/65913 Handle alignment in __atomic_is_lock_free

[Jonathan Wakely]

This fixes a 5/6 regression that causes atomic<int>::is_lock_free() to
require libatomic even on targets where the compiler knows the answer.

The new test only runs on x86_64-linux and powerpc*-linux. It isn't
actually OS-dependent but as long as it runs somewhere we should pick
up regressions so those commonly-tested targets are sufficient.

Tested x86_64-linux and powerpc64le-linux.

Richard, this is your middle-end change plus your suggestion for the
std::atomic members, which works just as you said it would. OK for
trunk?


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