[v3] <stdatomic> -> <atomic>

[Benjamin Kosnik]

... and the other changes from N3000, modulo the lock-free property
macro changes. Fixing those seem minor and can be done without filename
changes, include changes, and ABI shims.

Looks big, but is mostly renames. There is one ABI issue, due to
changed signatures, but it is relatively minor and can be worked around
with _GLIBCXX_ASM_SYMVER. 

Please OK for gcc trunk.

tested x86_64/linux
testing darwin in progress

-benjamin