[msandr] Access app TLS directly in native exec mode.

In optimized hybrid execution we do not use DynamoRIO private loader, which
mangles TLS access, so we can access the application's TLS directly.

Patch by Qin Zhao.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@199655 91177308-0d34-0410-b5e6-96231b3b80d8