Merge "x86/x86-64 read barrier support for concurrent GC in Optimizing." am: 3eb242c0c0 am: 1d8ed62fb4
am: af9955c5a3

* commit 'af9955c5a3dfa56960b98ab018842cc58dc6cd82':
  x86/x86-64 read barrier support for concurrent GC in Optimizing.