[ASan] Do not use kDefaultShort64bitShadowOffset on Mac, where the binaries may get mapped at 0x100000000+ and thus may interleave with the shadow.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174964 91177308-0d34-0410-b5e6-96231b3b80d8