[libc++abi] Replace __sync_* functions with __libcpp_atomic_* functions.

Description

[libc++abi] Replace sync_* functions with libcpp_atomic_* functions.

This is basically part 2 of r313694.

It's a little unfortunate that I had to copy-paste atomic_support.h,
but I don't really see any alternative.

The refstring.h changes are the same as the libcxx changes in r313694.

Details

Committed
efriedmaApr 16 2018, 3:00 PM
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