tsan: use read lock instead of write in atomic operations when possible


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204654 91177308-0d34-0410-b5e6-96231b3b80d8
tsan: reorder SyncVar members to reduce contention



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204655 91177308-0d34-0410-b5e6-96231b3b80d8