[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.



git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@330162 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed